Well I'll have to say that I wasn't a big fan of the first one. I loved the mechanics of the game; it was the atmosphere that I didn't like. It kind of had a Blade Runner (the movie) feel, and I'm just not a big fan of that environment. The rpg elements was a nice touch, and I do love rpg's. The story was kind of slow... and didn't interest me at all.
But in the end... will I buy this one? Probably. I love games enough to spend the money... keep the industry flowing.
Thief on the other hand, was one of the best games I've ever played. Definitely in my top 5. And I'm really looking forward to Thief 3.