Lots of fun, lots of potential, lots of bugs, lots of frustration
Comparisons between this game and Diablo 2 are inevitable, since it's the same team (and they've felt no reluctance to compare the game to Diablo 2 themselves). So I'll get this out of the way: If you enjoyed the compulsive item collection and random combat of Diablo 2, and don't mind a futuristic setting and a 3D interface instead of an isometric one, you should be able to get into HGL with no problem. Classes don't get as many skill options as they do in D2, but items are very diverse in abilities and the stat restrictions on them will have you doing plenty of careful gear juggling/modding as you level up.
So the core theory of the game's playstyle is tried and true. You enter a station and get some quests. You go into an instance and kill a bunch of enemies. You pick up a number of items and complete your quests. Then you go back to a station, break down/sell the stuff you found, and turn in your quests. Repeat. The level design is random, but frankly the tilesets wind up looking mostly the same. "Oh look, another brown underground level. Hey, now there's an outside level! But it's still brown. Darn." You get an occasional storyline-related level that has a unique look, but there are very few of them. It seems like the designers did put work into designing some varied tilesets, but then the urge to make each Act have a visual theme left us with multiple instances per Act, all looking too much like each other. In Diablo 2 it was easier to overlook because all zones were connected to each other - it felt logical that everything would look the same. In HGL, all instances are reached through portals, so they feel quite disconnected - and thus a player expects more variety from them.
Unlike Diablo 2 there's a subscription option. Where D2 needed an expansion pack to add to its content, and has otherwise received no additional content over the years, the plan for Hellgate is to add new content quarterly for those willing to support its development. We'll see how this works out in the long run. For now the game is new, so there's no real extra content, and the first subscriber-only event that's running now (Bonfire Night) feels pretty clunky and is rife with so many extra quest items that several subscribers are wishing they could unsubscribe just to keep them from dropping.
And there we come to the problem of potential: There's a lot this game could be doing wonderfully, but to do so it will take time and development. Whether or not it will ever realize that potential, no one can say yet. But at least the game they deliver right now is fun, despite the problems with it at launch.
And yes, there are problems. Several major bugs have been fixed (many of them carried over from beta), but several others remain. The randomly-generated instances sometimes have tunnels disconnected so you can't get to the end of the instance. Several quests are buggy, especially ones at the higher levels that saw less testing during beta. Some skills don't work as advertised, and there's a lot of balancing to be done between classes - some that seemed strong in the lower levels of the beta are finding it difficult to progress in higher levels.
There are also some glaring omissions from the interface and game features. There is no in-game mail system. There is no auction house or market. There are guilds, but they're essentially extended friends lists - you can invite, you can promote, and that's it. You can't add characters to your friends list unless they're online at the time (curiously, the guild system lets you invite offline players so they'll see the invite when they log in, a very nice feature). The chat window is on or off - no fading, no resizing, and no editing what channels you join or display on different tabs. The system to look for a group is an "autoparty" toggle that MIGHT get you grouped with someone else entering the same instance as you somewhere close to the same time, but generally yields no result.
The inventory system is similar to Diablo 2's at its initial release, but deserves a paragraph of its own. There is no autosort option, so when your inventory is littered with small items you've picked up and are taking up grid space that could be used by larger items, you have to reorganize them by hand. One nice feature over D2 for inventory is the ability to dismantle unwanted items for spare parts. Those parts are used to craft new items and to upgrade older items (to help weapons and armor with nice features keep up with you in level until they become too expensive to upgrade).
The story is about what you'd expect from a game of this sort - invading demons, stop them, etc. It's not a massive story, but I find there's enough meat there to satisfy me between zombie-slaughtering sessions, despite complaints from elsewhere about a slim story. One often-overlooked feature of the quest log interface is that you can get advice about quests from the companion you meet during the tutorial instance, Murmur. Murmur adds to the storyline quest text, fleshing the story out a bit and giving it more flavor. I assume they moved those additions to the story text to Murmur's option so that the people who don't care about story don't have to skim really long quest entries, but it's a very low-key option that's easy to miss until you're well along in the game.
The music is excellent, but plays infrequently and at seemingly random times (I've often had blood-pounding music start playing only after I'd killed a large group of monsters and had nothing left to fight). The sound is good, making enemies recognizable before you see them.
My system doesn't let me test the graphics to their limits, so all I can really say on that score is that if you have a lesser system (1.8 GHz Athlon here with a 7600 GS), you can drop the graphical options and still have a nice-looking, smooth experience.
So again, my take on the game: It's fun. If it sounds like your cup of tea, buy it and play it. You'll run into bugs, but most of it is a good time, killing demons and taking their stuff. The developers have made a lot of promises about more content and features, so if you're easily frustrated by a game crash that makes you start an instance over, wait a couple months before picking Hellgate up to see if they deliver the bug fixes and additions. If you're thinking about subscribing, don't bother yet - their first content patch is scheduled for December, wait to see how the early subscribers react to it before spending more money on the game. There's plenty of non-subscriber content to keep you busy until then.