When you need to bring John Stamos in to revive your medical drama, it might be time to announce it dead on the table. E.R. finally threw in the towel after 467 seasons (approximately) and very little of the original cast still involved. They actually had to bring in actors to replace the actors hired to replace the original actors.
With a tepid final season and frustrating conclusion, Battlestar Galactica took a big slide in quality. The early seasons were some of the best science fiction ever made, but it suffered (as did Lost) from the mistake of trying to inject too much mystery and religious symbolism into the story. The show was awesome when it was about killer robots and political intrigue. Not so much when it was about chasing a little girl around an opera house and being visited by angel-ghosts.
Much like Lassie, Entourage suffered from showing the same scenario over and over and over again. Vince gets in trouble. Then Vince gets out of trouble and all conflicts are resolved. Then again, Lassie was pretty lacking in the scantily-clad model category and Entourage excelled at that. I thought dogs were supposed to be babe magnets.