I just finished Season 4 
Prepare to have your love dimmed in S5.

Is it the worst season? Or just a couple of notches below the awesomness that is Season 4?
Its
easily the worst season. I wont spoil anything for you, but for a show that had up until that point been fairly creative and fresh, they use some of the worst plot devices and lazy writing techniques the show has ever seen in S5. There are a few good episodes, but the season as a whole, and the main thrust of it, is so insanely distinguished mentally-challenged that it hurt my overall love for the show.
Do you like time travel? How about 17 episodes of it? Do you like when amnesia is used sloppily as a plot device? If your answer is no, then youll hate S5. Also, they fuck up the character interaction of the show too by splitting people up. Do you love all the Jack and Locke conflict? Guess what? They share ONE scene in the entirety of S5. Also, if you're a big Locke fan, prepare to be buttfucked.
Anyways, yes its the worst one. Especially disappointing coming off the excellent S4.
S4 > S1 > S3 > S2 >>> S5