I've watched BtVS regularly since the very beginning. (I missed all of I think 3 episodes in the course of its 7 season run, and I caught those in reruns/DVD.) I don't tend to rewatch much, but I've seen the musical maybe 10 times by now (the bulk of which immediately followed its airing -- literally probably 6 times in two weeks). I inflict it on everybody and while the resonances are huge if you know all that came before (and comes after) I really think it works as a stand-alone.