Supernatural's a fantastic show - I've followed it religiously since it first aired despite receiving criticism from everyone I know for 'watching that tripe.' The season premiere was excellent; uncomplicated, forthright and simply top-notch entertainment. Some shows leave you waiting for years before they answer pertinent questions, but Supernatural's clever in that it doesn't beat around the bush; it simply keeps things flowing, answers recent questions and asks new questions. As in every show I prefer the standalone episodes, but the demon arc that has developed throughout has proven to be very, very good too.
But yes, the chemistry between the characters is what makes the show. Ackles and Padelecki have formed a believable brotherhood between them, whilst a character like Bobby is very good glue for forging links in the stories. The main actors are all really talented, even if the tag-alongs can at times be shoddy.
Favourite episode has got to be either the one where Dean keeps dying or where Sam has all the bad luck after losing the rabbit's foot (Bad Day at Black Rock, I think the latter was called).
Scariest episode is probably the Season One episode, Asylum.
I've got the first three series on DVD, but am a bit peeved by ITV's poor coverage of the show. It takes them forever to get the new series after the Yanks, and their advertising is non-existent, yet still they whine about poor viewing figures. Add to that they've messed around with the repeat (original airing time of 9pm on a Sunday on ITV 5 million is not ideal), and it makes for a dreadful attempt at bringing a fantastic series to a wider audience.