I should be working but...
Is it better to end a TV series earlier than expected, but while it's still good, or let a series run and run, until it is eventually axed?
Exhibit A: Heroes
A good premise, and to begin with it was well executed, but the writers seemingly didn't plan further than the first season. Although it could be said the 07-08 writer's strike was the first nail in the coffin for this show, from the second season onwards the plots seemed very thin on the ground and characters switched sides more times than Nick Clegg.
Exhibit B: Supernatural
Unlike Heroes, this show was created with a story arc planned to flesh out over five seasons. While this was the case, with fans of the show generally accepting that those five seasons came to a satisfactory conclusion, money talks and the current sixth season which follows a new unplanned story arc, seems somewhat out of sync with the rest of the show. It's not been axed yet, but only time will tell, how long they can keep milking the original premise.
Exhibit C: Lost
Like Heroes, there was no planned story arc past the first season of Lost, instead it took the writers until season three to finally plan out the overall plotline the entire show would take. One could say this saved the show, which somewhat lost its way mid-season three, but despite planning the ending, fans felt somewhat cheated when the final season ended with many questions left unanswered. Perhaps if they had spent seasons 4, 5 and 6 answering those questions and not asking new ones, the ending might have felt more satisfactory.
Personally I believe that all three shows had/have their merits, but that they also didn't/don't know when to say stop.
Heroes' first season is one of the best opening seasons of a show I've ever seen; there wasn't a single duff character and they didn't ask too many questions, that they weren't able to eventually answer. But it should have ended there, instead of trying to add too many new characters, and too many new plotlines; it just overwhelmed the show.
Likewise, Supernatural's first five seasons came to a satisfactory conclusion and if the show had ended there it would have felt right, but this current sixth season is just milking it for all it's worth, and has totally lost track of the original vibe of the show.
And Lost, well Lost is a bit of a conundrum; I simply don't know why, when they had planned the ending, and had decided it would end after six seasons, that they didn't plan it so that every question could be answered. The ending, felt perfect, ridiculous and unsatisfactory all at the same time. I'd rather they had given up after Season 3 than spent three more seasons asking more and more questions that they were never going to answer!