So I'm gonna go against popular opinion here...
I'm not big on the whole stealth games thing. It adds variety to missions if occasionally you are required to use stealth to get things done, instead of just happily shooting/slashing/bludgeoning enemies into submission, (eg the stealth missions in Vampire: The Masquerade - Bloodlines add a different challenge and are a lot of fun). Despite not really enjoying stealth orientated games I was prepared to give Thief a go, just for a new challenge and the fact that I remembered my mate almost having an accident in his pants telling me how good it was. He was 14 at the time, so such an accident would not have been forgivable.
So I gave the game a go, have persevered for quite a while with it, and have tried to get into the style of the game. But yet it stands now as the only story driven game I currently own that I haven't ever completed. This isn't because I find it too hard, it's just that it bores me. I'm a patient guy - I'm in to point and click adventures, for goodness sakes - but I just get frustrated with Thief. The plot moves slowly, the missions don't really seem connected, and the whole thing just doesn't engage me. I just get the feeling that it's a good concept and it should have been done much better.
Also it is pretty bad graphically. People and objects seem the wrong shape at times, some items are proportionally incorrect in terms of size, and too many of the levels are very plain. The character that you play as, Garrett, seems clumsy and is awkward to manoeuvre, especially when attempting to knock out enemies.
It is good in terms of sound, and it is interesting to have to make your move often just using sound as a guide for whether an enemy is nearby. This is an interesting concept, and the sound is done well.
I am yet to experience any bugs with the game, but then again I've only ever played it halfway through, so there may be as yet undiscovered corruptions lurking within. To be honest I can't see me ever learning if the game is totally bug free.
So in all, this is an awkward, slow paced, disjointed, graphically below par game. It is a good concept however, and maybe the follow-up games in the Thief series are better. I hope so. I want to be able to like this game as it does have a few positive elements and an interesting concept, but sadly far too much counts against it. If you are able to persevere with the slow and at times unconnected story then I'm sure this will be a rewarding experience for you. The only thing I feel for it is disappointment.