I love OMWF. True, the plot isn't that great, but the over-arching story has necessary and important movement in this episode. As is traditional with musicals, this is found in the songs. The purpose for the episode, the brilliance of the episode, and the necessity of the episode occur within the lyrics of the song. This one episode captures so easily things that could otherwise take a season unto themselves to come out. Things such as Giles' decision to leave, Tara discovering Willow's spells, everyone finding out about Buffy being in heaven, and the like. These aren't things that can happen overnight under ordinary circumstances, but because of this episode, the rest of the season can get underway and get on with the main plot of things.
Not to mention the music is just catchy and the songs stick in your head easily. 😉