I remember really enjoying Heroes when it first came on, but I think I mannaged to watch two seasons before I lost track of it. Not because it went bad, but I just lost track for some reason.
I have heard that the longer it ran on many mistakes were made by the writers and it got progressively worst, but I can't make an opinion on it as I haven't seen enough seasons. There's a video about the flaws that it had on YouTube though, but obviously it's this one guy's view, but he makes sense.