It's amazing. There's so much to do beyond the main quest, which is huge. There's different guilds, each with their own questlines (all of which would be worthy for a game of their own), plus stand-alone quests (of which there are hundreds), and even if you do no quests of your own the place is feckin' HUGE, and there's so much to explore and kill and loot.
I would, however, recommend you look at downloading some of the community-made mods (if you have the PC version). The levelling system kind of sucks, and those fix that, and they also add even more content.
My one gripe with the game is the scaling system, where no matter what level you are the enemies are equal level. This means wherever you go the enemies are equal to you, so levelling up doesn't really matter that much except for getting better loot -- the enemies level up with you. Again, though, community made packs fix this.
So, I would recommend if you got this game (which you should), you get the PC version. There are some flaws in the game itself, which, with the PC version, you can download mods to fix. You can't do that on the console versions, so you're playing more flawed version, with less content.
But it is amazing. So much depth, so much fun, so much variety. You can complete it as a stealthy rogue, sneaking around picking people off with a bow, then a huge warrior, charging in and hacking your enemies to pieces, then a powerful mage, summoning demon familiars and blowing people away with awesome spells, a combination of those or even something entirely different.
It's a fantastic game. Buy it.
