They were fairly hit and miss with me gameplay wise.
Clickers are intimidating a lot of the time, especially when mixed in with regular Zombies. The first proper encounter with them and the zombies in the skyscraper is perhaps the most intense feeling I've had in a stealth game, and still grips me whenever I replay it. Usually, however, it's just really tight rooms and an over-abundance of them, leading to a game of tedium so I can just get past them. There's the odd good moment with them like the dark sewers, but that's it really.
Story wise they raise questions that Casual Shinji already brought up, but I do admit that I like them conceptually. I'm not sick of Zombies, so seeing new types of them always feels fresh with me. The Bloaters can fuck right off, though. They look and act like the Gunkers from "Gears of War 3", they're just as annoying but nowhere as satisfying to kill. The regular Zombies are fine, and I like the "Left 4 Dead" influence in them (standing around crying, the AI is programmed so they have different reactions to sound etc.)
EDIT: I've noticed this was necro'd, but still, that's my thoughts on the matter.