I fell away from it about mid season 6, but only because I moved to another area where it wasn't on TV. I dithered over whether to buy the DVDs, because I wasn't sure if I really cared all that much anymore. But I went ahead and bought them. I enjoyed season 6, season 7 had some ups and downs. But I didn't regret it to such an extent that I'd stop watching. I'll just have to wait for the season 8 DVDs again.
I think Supernatural is a show that could benefit from a short cable season than a full network season (plus being on the CW doesn't help either). There's a lot of fat on this show that could be trimmed down. They like stretching things out to the point where the apocalypse or leviathan invasion seems like something they will fight on their spare time.
When the show started the producers delved in legends and lore that seems to have worked for the show,...what kind of series would work if things ended in one season? Stick with it....Just remember it should have ended after Sam fell into the pit...so far I have not been let down.