I've always really liked Crysis 1, and Warhead was excellent, even better to some extent and in some places than the original. I've also always liked the zero gravity alien level, I thought it was very well executed and the first time I played it the sense of disorientation was near perfect and the environment was very much alien. I know a lot of gamers out there even those whom happen to appreciate Crysis 1 overall actually despise that specific level anyway, but I never did and always thought it was great.
I've only recently bought Crysis 2 via the EA Humble Bundle (with Dead Space, Red Alert 3, etc) and even though I've played it before with my cousin on his 360 it was only two short sessions (when he happened to rent it), so I can't judge the sequel too much for now (but I've read a lot about it and I do expect a corridor-style shooter). As for Crysis 3 it seems at first glance that CryTek attempted to bring us back to the "roots" (jungle, overgrowth on ruins, high grass, more open-world, etc, visually more "Crysis 1-style") so perhaps I'll enjoy that one more than the sequel, time will tell, but I don't own it for now. I do plan to buy it eventually, however.
In my humble opinion Crysis isn't a bad FPS franchise at all, there's definitely worse, and I think that only the sequel's first impressions by many (perhaps before they released the HD textures) perhaps contributed to "reduce" its weight and quality for the genre. Just a guess, maybe it has nothing to do with that, but the "consolization" of the sequel was practically a headline when the game was freshly released, not to say a controversy since the original set new visual standards, so why would the sequel be a "step back" (I do remember heated debates related to that).
Ultimately I'm pretty sure that all games in the series so far (including Warhead of course) have their good moments.