I have it, played it for a few hours last night. It was fun.
as far as how it works, it's more diablo-ish than it is MMO. everyone connects to the same virtual game world and you can see/interact with people as you like and form up parties. Once you leave a town/settlement type area, though, the game "instances" out the game world for you/your party. So if you go solo-ing, you won't see any other real people, just NPCs and the like. When I was fiddling with the beta events, though, the game really picks up once you hit the level cap (lvl 20 which isn't too hard to get to at all) and you do things like guild wars, and arena matches and the like.
If you're looking for an MMORPG, you might want to just pick up WoW tho. This is more like co-op PVP type game with an exceptionally fancy matchmaking/character creation system when you boil it all down. Good storyline tho.