I loved it during beta, but since release for some reason I found it got boring quicker than I'd hoped. Luckily there's no subscription so I can go back whenever I feel the urge (might not be for a while as I've just started Borderlands 2). I still feel it was a worthy purchase.
Issues for me include (mainly a guess as I can't really define why I found the game boring so soon):
Combat isn't deep enough. Don't get me wrong I think the combat engine is very solid. The way your movements and positioning have an effect over the outcome of a fight is leaps and bounds ahead of WoW and WoW clones. Mobs don't stick to you applying damage passively, they take concrete swings you can avoid. Kiting feels like a proper general mechanic rather than an exploit. Melee combat even involves a certain amount of kiting. But the class designs and the abilities you get to choose from make the combat feel more repetitive than it should do. Weapon skills feel like they were cobbled together with no thought to their interplay. In most situations the most appropriate strategy is to push each and every button the moment it becomes available, a.k.a. whack-a-mole. Utility skills are predominantly trinket style cooldowns (which is what you'd expect from the elite skills) rather than general purpose tools. There are even utilities with passive components that essentially reward you for (or at least give you a valid option of) not using them. Given that they made the decision to limit the number of buttons how they did (which wasn't necessarily a bad move), there needed to be much more emphasis on activity rather than passivity. With such a great engine there could have been more emphasis on movement, stuff you have to aim, abilities with different shapes. Basically, stuff you have to think about using. There are examples of that, but not nearly enough.
Lack of progression. You have most abilities learned by level 30, and probably all except the two most expensive (which are likely rubbish) by level 40. After this, it's just passive stat increases. Traits are almost entirely passive. So actually, they didn't even get rid of the progression mechanics, they just got rid of the fun side of it. They got rid of the active kind of progression where you unlock fun new skills and modify them to do new things, and kept the boring passive stat increases. The loot is similarly passive. Every item is just a bunch of stats when, given the engine, they could have done so much more. How about a set of boots that modify the effects of your dodge roll? I'd do dungeons for that. How about different ranges and shapes on weapons of the same type, to emphasise certain abilities over others? Progression doesn't need to be so dull. Instead, all I get is a choice between 10 Power and 10 Precision.
Group play is incoherent. I don't think I need to elaborate much. It's good that the trinity is gone, but it needs a replacement. I'm not convinced that the dungeons can be balanced correctly. Sometimes the difference between feeling cheap and feeling too easy is the difference between being one-shot and not being one-shot. They can't fix it with a numbers tweak. They need group mechanics built from the ground up. All I can think of to fix it now, with what they have, would be to introduce some sort of artificial mechanics via the environment weapons on a fight by fight basis. You'll have to wait for the next big MMO to see a fully fledged, organic, non-trinity based group dynamic.
But it is a good game, and worth a single purchase IMO. The levelling process is interesting enough, even if the endgame is lacking. The scaling problems with the DEs only happen right at the start and right at the end, so for the bulk of the levelling content they work pretty well. The PvP is the best I've ever played. The story... well yeah it's shite, but I actually really liked the story quests. Ignoring the bugs, the quests themselves are well designed mechanically, with good pace and balance. You've got lots of mobs, lots of NPCs, like you're just one person taking part in a full scale battle. Reminds me a bit of reclaiming the Undercity in WoW, which was easily the best solo content they ever did. It's a good game, but I can't help but feel it could've been a lot better. Unlike with earlier MMOs, I don't feel like it was a restriction in the technology. They could've made a much better game based on the exact same engine, which is a bit sad actually.