Supernatural is a great series in general - one of the best written shows on TV.
That being said, I'm pretty sure the original intention of the show was to end at the finale of season 5. They'd decided to continue the story on, and this is why I think this season takes a few more episode than normal to bring you in.
I will say most of my friends who were fans had some issue staying with the series through this build-up (even I started having a doubt or two) but when you see where the story is going and the truths behind what's going on come out, you'll be glad you stuck with it. The slow build had everything to do with a big story payoff and nothing to do with poor writing or plotting.