I thought it was the best in the series, though it didn't have the impact of 1 and 2. Mostly very good level design, VERY good visuals, nice puzzles, and an incredible soundtrack. The storyline was actually pretty interesting as well...
Problems? There are glitches in the game, though I didn't encounter many (only 1 major one involving a trailer). The rest of the game worked perfectly for me. Many people seemed to have issues with the controls, but when you learn them, they are much easier to use than the previous games (the steady walk is VERY useful). I'd also say that the first few areas in the city are a bit dull and broken up by way too much loading.
I really had a good time with it for some reason. The new elements are a bit weak, but they don't really hurt the game...
I'd say buy it...but play with an open mind. Too many people gave up right away and ended up hating it.