Many times the Supernatural series have been described as being either about the brothers or about hunting monsters. I have seen all episodes through all eleven seasons until now, maybe just out of habit, or because of a gem or two once in a while.
I like the good episodes and hate the bad ones.
There are very high and very low moments in quality.
Hence, I believe hard-core fans, which I feel are many by public acceptance and imdb score, do over-rate a bit. But also the non-viewers may be missing suprising elements too.
Hunting monsters is very little valuable, they mostly have very little plot, and are just kicking asses and eating, drinking, banging, seducing nice stuff. Once in a while a character is presented and/or some development on secondary characters or main ones. This isn't always painful but the most interesting catch of the program may be another side, to which I argue in favor.
Suggestions for watching some parts
Season 4-5 & 11 have participations of highest(/lowest?) spiritual entities known to most mortals:
God / Lucifer
And they get to develop very interesting dialogues and leave a lot to think of. That alone I believe is enough to make the show valuable and forget the rest. Other topics/caracters that deserve attention and may appear on other seasons: Reapers, Death, The Four HorseMen, Cain's Mark, Djinns, Groundhog day experience, a cursed rabbit's foot, etc. For reruns, I'd be pleased to see those seasons and several episodes again. The rest, well... I'm giving forward notice, so the warning is there.