I've watched filming as an extra before, and while it can be very boring, it can also be fascinating. Especially, I find, as an extra who moves from set to set... you get to see the different sets and how they function and feel.

I've only watched SPN film for an hour or so in the evening - and it was ridiculously entertaining solely because of Jared having fun. But I also did see what Russ was talking about, when everyone just grabbed a piece of equipment and walked the whole show 100 meters up the road and had everything set back up again and ready to film in 10 minutes.