I own the first three seasons on DVD but have only seen the first two so far. I think the Writer's Strike really hurt the show, as the second season could not hold a candle to the first one.
I think a problem with Heroes is that each season(or even "story") was supposed to deal with new heroes, thus keeping the stories fresh. However, everyone seemed to love Sylar so his character stuck around(even though it was rumoured he was supposed to be killed at the end of Season 1). Season 2 had the addition of a couple of very weak characters(the woman with the killer tears and her brother that prevented it? That was bad.).
I will still pick up season 4 when it drops down in price.
Also, it looks like there is going to be a mini-series airing on NBC some time in the future to tie up any loose ends. That should be welcome news to fans of the show.