I feared for both Lost and Battlestar, but I think they've both come around. Having ending points for the shows helped, I'm sure. Wish I could say the same thing for Heroes.
If they go on long enough, every show jumps the shark. Some of my faves, Buffy, Homicide, hell, ever the Simpsons, get weaker as they go on. I wish we would adopt a style closer to that of the BBC, where shows have a 6-9 episode arc, and then they go away for awhile. It would allow for more variety and stronger writing.
However, Arrested Development never jumped, except is a cool exteremely funny way.