I don't have any way to watch TV, but I did net flix the first 3 seasons of this show.
Season 1 was absolutely stellar, and this is coming from someone who's opinion of television is so low he doesn't have any way to watch it ;-)
Season 2 was quite bad, season 3 picked it up. The characters are getting very stale, and Hiro and Ando, my favorites from season 1, are walking repetitive jokes now.
I think the reason season 1 was so good, and the others are weakening, is because it was a really good story of people coming to terms with their powers. The audience was constantly introduced to new powers, and we didn't know how they would evolve. The characters had very real problems, the plot twists were genuine (Bennett turning out to be a decent guy threw me off). Sylar was also a consummate villain.
Since then, it is a lot harder to write characters that already know their abilities. Almost out of necessity the show has to become more action oriented, and that doesn't work as well on TV.
And if they would get rid of the preachy, failing-at-sound-deep monologues I'd like the show a lot more.
Lastly, I agree with whoevever said the time travel doesn't work. It's a gimmick and it is such a genuinely complicated idea it can't work in a show this light.