Judging from the beta, it's not a game that will knock your socks off, but it is cool enough that I kept my pre-order. Occasionally, gameplay will feel repetitive. But at other times, it gets truly intense and offers some gameplay elements you don't see very often.
On a scale from 1-10 (where 5 really does mean mediocre, rather than bad), I'd place Hellgate: London somewhere around 7.5 or 8. It's worth the money and will offer some experiences you won't find anywhere else, but it can feel redundant. If you tend to make a lot of alternate characters, like I do, try to delay your alt-making as long as you can. You'll enjoy the game more if you keep pushing through.
But it's not a game for everyone, so I recommend reading on to see how it suits you personally.
It's not an FPS game, but it's close. It's close enough that, if you're an FPS fan (like I am), you're likely to be frustrated initially by the stats-based auto-aiming. It's particularly frustrating when you get your hands on a more realistic weapon, like a sniper rifle. A sniper rifle is one of my favorite weapons in any FPS game, so not being able to zoom in for a careful headshot is torture.
But I got used to it after a while. I bet that many gamers won't be able to set aside those expecations so easily, so please don't take this as a promise that you would get used to it. It's part of my personality that I'm quick to accept whatever comes my way. If you're not that kind of personality, you might never be able to get past your true-FPS desires.
The right-click menus will take some getting used to as well. You don't right-click to pull up a menu, then left-click your selection. Instead, you right-click and hold the right mouse-button down, move to your selection, and let go.
Overall, the user interface is familiar. It's nothing you haven't seen a dozen times before. The non-combat action you'll be using most often is dismantling items (instead of selling them).
 The game can be twitchy, if you want it to be. You can dodge fireballs and other missiles. You can sidestep some attacks. Timing your blows and movements can make a big difference. For example, those little tadpole guys zap you after they're dead... but they won't if you kill them quick enough, by closing the distance and timing your blow right. When those leaping dog-like demons leap for you, you can time the swing of your sword so that you catch them right before they land on you. There's also some environmental play that's based on timing. You can maneuver so that demons have to run by barrels to reach you, then you shoot the barrels as the demons run by and they explode. Positioning enemies can often be a big factor in combat.
Hellgate actually has a lot of depth to its action that's only noticeable if you take advantage of it. But if you want to play more casually, you can do that, too.
Demons and Combat
Flagship did a fantastic job in coming up with imaginative enemies, each type with a unique AI personality and impressive visual design.
Just in the first ten levels (minor spoilers), there are snipers which try to keep their distance, flying demons which swoop in and then back away, charging berserkers (some with the ability to stun), slow creepers which will breathe fire on you if you wait too long to attack, little tadpole-like creatures which will occasionally morph into something much larger and formidable, stitched-together monstrosities that release other zombies and bugs upon death, demon dogs which leap at you from far away, monsters which burrow into the ground and could pop up anywhere, and on and on. Like I said... that's just the first 10 levels!
Flagship's creature design is a wake-up call to other developers. I often think of what a great true-FPS these sort of enemies could make. One demon has a quick stride that sends him leaping from side to side as he charges at you. In a true FPS, trying to aim at a creature like that would be difficult, and could force the player to strategically change tactics or weapons (such as to a shotgun, which has a wider spread). In a true FPS, those dog-like demons leaping at you would be even more fun.
Hero monsters are back! Occasionally, you'll be surprised by a demon with a "rare" or "unique" name color, signifying that this demon is tougher than its brothers and may drop some especially nice loot if you kill it. This is the sort of dynamic content I was really hoping would make its way over from Diablo 2, and it did!
The demons wander constantly. You often have to pay attention to what's going behind you as well as shortly off in the distance. You might still be trying to fight a few demons when more wander into aggro range and join the fight. And many of the levels are cleverly designed so that you have to enter hallways or other areas with demons on two sides (i.e., turning your back to an enemy is, at least momentarily, unavoidable).
You're almost always fighting multiple enemies at once. Sometimes, you end up with more than you bargained for, fighting so many enemies at once that you're completely sucked into the game and tense with adrenalin. When you catch yourself sighing with relief, you know you've just had an intense gaming experience. For lovers of intense action, I recommend starting with one of the pet classes -- the Summoner or the Engineer (especially the Engineer, since their drones are ranged fighters). Blood-pumping chaos is the only way to describe many of my Engineer combat experiences.
There's something missing from the melee classes. The strong kinetic feeling just isn't there. I don't feel like I'm really beating the hell out of a demon (pun intended). No matter what I'm hacking on, the sound effect and animation is the same. I swing my sword, the enemy dies, but I don't feel a strong connection between the two.
The ranged classes, however, and the pet classes in particular, are a lot of fun. While I have to admit that I'm a little disappointed in the skill selection, in that skills don't quite match the variety and fun factor of Diablo 2's skills, Flagship has repeated Diablo's success of allowing players to customize characters into unique representatives of their classes. As an Engineer, you might concentrate most of your skillpoints on improving the armor and firepower of your drone; or you might enable your missile bot to fire more missiles; or you might empower your inhibitor bots to slow enemies even more; or you might power up your focused attack. Between loot and skill choices, there will be considerable variety among players of each class.
Visuals and Audio
Like I said, the creatures are well-designed. But I agree with Bildo that the levels often look too similar to each other. The interiors of the levels are usually different from one another, but the buildings which form the borders are the same. This definitely affects the player's enthusiasm, but the new demon types, weapons, and skills are enough to keep things interesting during progression.
Equipment looks great. All gear that I've seen looks pretty cool. Weapons, too. I wish I had a screenshot of my Guardian's glowing-rune-covered cricket bat. When ranged weapons are upgraded with fuel canisters or ammo clips, you actually see the upgrades on the weapon outside of the inventory screen. Flagship cleverly included a "vanity cam". Just hit "V" on your keyboard, and the camera will slowly turn around your character.
Many players will be irritated when a new helmet completely hides his or her character's face, but this is more than balanced by the ability to apply the color of one donned piece of equipment to the entire set. So if your boots are natively blue, your gloves are red, and your helmet is black, then you can choose to apply any of those color schemes to every piece of equipment you're wearing. Wisely though, the selected color is just the basis of the color scheme. So if you chose brown, your character's clothing wouldn't be entirely brown, but would end up looking something like this.
The sound effects are better than videos had led me to believe. Sometimes they're great, but sometimes they're mediocre. Reverb is applied universally; so there's a slight echo everywhere, like you were always fighting in a tunnel. I assume Flagship's audio team thinks that adds to the tone of the environment, but I think it was foolish.
The music is alright, but not wonderful. The trailer music is better than most of the in-game music. Honestly, music doesn't play much of a role in Hellgate. I rarely felt like the gameplay was more intense or otherwise appealing due to the music. The sound effects are far more important to this game. I wish they had contracted Matt Uelmen, who composed the outstanding environmental music to Diablo 2.
There's a lot of humor. Some testers think that it's too much, but I enjoy it. The lore setting is pretty grim, but Flagship doesn't take it too seriously. You'll meet madmen, a vendor who cusses like a sailor and openly admits to selling junk, a woman who's constantly trying to seduce you (regardless if your character is a man or a woman), cowards and wimps, and many other funny characters. There are some serious characters as well, but the overall tone of NPC dialogue is meant to make you smile. So far my favorite character is the madman in the very first level, who is constantly muttering nonsense like "Twelve galaxies, and all coming at me!" and "...but that's not how much milk goes into scalloped potatoes, sir!".
Subscription or not
Bildo says that non-subscribers will get only 3 character slots. That's definitely annoying. Diablo 2 offered more than that, and that game's character classes were more different from each other than Hellgate's. As an explorer type, I make alts all the time. Right now, in beta, I have 6 or 7 characters. I'll miss that.
But I still don't plan on buying a subscription. They'll have to convince me of the value.
I've you've ever been to London, like I have, that will act as a bonus to your gameplay experience. I've been to the British Museum, so being able to fight in a simplified but easily-recognizable version of the museum was a thrill. If Flagship makes an expansion or sequel to Hellgate: London, then I hope they pick a new city for the setting. It would be great if everyone could experience what it's like for a real setting with personal relevance to be included in a game and refashioned with an element of fantasy.
Since I've been in the beta for about a month, I don't feel the need to try the demo. But I strongly recommend that anyone who is considering the game judges it from the demo and not just from videos and preview articles. Much about the game can't be captured in any way other than by playing it. Honestly, the videos I had seen made the combat seem pretty bland and low-energy, so it was only by playing the game that I realized how intense and dynamic it could be.
I'm sure there's other stuff I could talk about, like the crafting system, but that should be enough for you to know whether or not you want to try the demo. If there's anything else you want to know, just ask.