Heroes: I loved this show in the first season, thought it was okay in season 2. But somewhere in season 3 I got so confused with the different timelines, that I realized I no longer cared. And stopped watching.
Bones: I loved the early seasons. But honestly the show went down hill after what they did with Zach :(. While I continued to watch a few seasons afterwards it wasn't as good. I stopped watching somewhere in season 6 I think.
Smallville: Early seasons were great. But by season 7 I realized the only Character I still liked was Chloe (Lois and Jimmy were okay). I stopped watching for a while. But I have seen until season 9 now, I doubt I'll ever finish the show.
As for Charmed and Gilmore girls. Both shows had a bad last season (both of which I watched hoping it'll get better). In Charmed's case the last episode was okay, not the best but I remember being a satifing finale. Gilmore Girls got better once Lorali broke up with Christapher and the mess with the new found Luke's daughter was behind her. I honestly didn't mind April, it was that there was no reason why Luke won't have told Lorali about her, since the relationship happened before he met her and he honestly didn't know about her.