First there was Thief: The Dark Project
Then came Thief II: The Metal Age, which kicked absolute arse.
Thief III: Deadly Shadows was... okay...
Which would make you think that the series has peaked and it's on the way down. But I hold high hopes for
Thief IV. The game has been in early development since May 2009, and while rumours fly around nothing shoudl be pinned down until Deus Ex: Human Revolution wraps up its development stage. Thief represents the pinnacle of the Stealth Genre, which has contributed to both its success (as opposed to bang-bang-shoot-em-up FPS) and the downslide of T3 which some (mainly those used to Halo, etc) gamers just didn't 'get'.
Also, I was sent a funny little review by Zero Punctuation on Thief. Play the game (I still have T2 and it runs well on XP if you want to borrow it) and enjoy:
Zero Punctuation: Thief