When Lost first debuted, I was a huge fan, but I lost interest (hehe, no pun intended) about half way through the first season.
Right now I'm watching Surface, Invasion, and Threshold and I feel the same way. They are interesting now, but I think as they drag on I'll lose interest.
These types of shows would make great mini-series, but I don't think they can sustain themselves as long term story lines.