Seen Religulous a few times, it's one of the things i put on sometimes in the background while I'm working. I could listen to Bill Maher read the phone book.
Re: Serenity. I actually saw it without knowing anything about it or Firefly, then later checked out the series. - The only problem with that is knowing the fates of certain characters, but I followed the movie fine without needing a bunch of awkward exposition shoehorned in.