The first season was good, and the second took it up a notch, while the third season was ok, which I think it took a bit of a hit, because that was during the time of the writers strike, and I know it only had a total of 16 episodes. The fourth was fair; it had some good moments. The fifth season was meh, and I really didn't care much for the ending. I mean, for the most part, you know it's going to happen the way it does, but there just isn't as much bang on the way out.
I have yet to watch the sixth, as I usually am away when the show comes on, so I end up buying the dvd sets when they come out. That said, I did catch one episode, and while it had some decent humorous moments, the face off with the supernatural being was somewhat lacking.
It's a good show, really it is, but I don't think it has got the same feeling now as it did back in the beginning, and they may be due to the fact that it was only originally planned for three seasons.
Still, I'm a fan of the show, and I hope it has a decently long run. =)