2-4 isn't unwatchable as much as a disappontment compared to 1.
(And season 1's finally, to me, was really disappointing...more so than several episodes of 2-4)
Seasons 2-4 had some good ideas, but not reaching season 1 heights.
Some of the ideas i liked -- the "old" gang of Heroes and they're plans vs. the next generation
The season 3 stuff with Sylar (was it 2) seemed kinda goofy.
Season 4 (like season 4 of Andromeda & Earth Final Conflict) was getting back to what we liked.
Robert Knepper did a great job of building a manipulative villain...it felt like his motivations/skills disappeared in the last 1/3 of the season or so .
if remember right, Sylar teaming up with Peter was interesting
jack cole of course does great as HRG, transforming him into a conflicted hero
definitely the last episode should be "essential", and absolutely the last 5 minutes, as the series wraps up like it began (like Lost's last 5 minutes).