Every season takes a somewhat different tone. The general consensus is:
Season 1: The best
Season 2: First half too many cliches, second half pretty good
Season 3: Fine but unmemorable
Season 4: Strong finish
Season 5: Wait, why is there a season 5?
Yea, I enjoyed all of the first 4 seasons pretty well. Boone's replacement for me, was a fine replacement, and didn't do any damage to the show. The next major cast member replaced, I actually prefer the replacement.
Season 5, was a bold decision for a twist, and if well executed, I think it could've been really great. The execution, however, wasn't so great, and S5 definitely suffered for it, but, I was able to find some stuff to enjoy in Season 5