I bought it, and have played it for about 9 hours.
Storm pretty much gave the basics of the game.
The music is pretty good. Though, I think the way I'm playing this game is limiting how much of the sound is coming through -- I have my ps2 hooked up to my tv card, and at times the music is "fuzzy."
The graphics aren't the best in the world, but I'm not expecting such in an RPG. On a related note, the camera takes a bit getting used to. At first it'll seem like it really sucks and if you don't adjust within the first few moments of the game, you'll probably just want to stomp on the game a few times because it'll piss you off. Even after you get used to controlling the camera, it still isn't a very pleasant thing.
Gameplay is simplistic. You walk around, hit the X button a bunch of times and defeat your enemies. If you dare go up against harder enemies, then you simply have to start using magic and all the neat scrolls of magic you find. It's also a nice way to level up. The area you enter has monsters at a cetain level, and YOUR level determines the amount of experience you get from the monsters. 1 level = 1000 exp. So if you're sitting at level 30 and you go into a level 1 area, expect only 1 exp/enemy. The nice part about this system is that it's easy to level up weaker allies. Everyone in your party gains experience, whether they participate or not. So if you bring a lv 5 character to a lv 20 or so area, expect them to gain massive amts.
Eventually you probably will get tired of the dungeons. This is where a lot of the gameplay takes place, and after you beat the game (which won't take long), the many "extras" you can go to are different areas generated by random "keywords". Each area has a dungeon and a few other things.
At this point...if a game was only like this, it would be less then mediocre, no? What makes .hack neat is being able to switch between the real world and "the word". You can check your e-mail, read the forums, communicate with your allies via e-mail, etc.
You might want to rent it before you consider buying it.