This was simply an excellently done movie, for it's genre. We're not talking Citizen Kane or Bridge over the River Kwai, of course- but it's not trying to be that. It's just fun, and incredibly witty. One of the best comedies I've ever seen, revealing of the drive for materialism verses things that really matter, full of Christological and soteriological allegory. Later on there was a TV series by the same name, that took the kernel of the idea and twisted it, taking something humorous and making it dark, new-agey, and frankly weird. This original movie has little in it to offend, unless, of course, you're a vampire. It's very vampirist.
Buffy cares only for herself, learning the values of the American Me Generation. Until she learns that she has been chosen for something far greater, and that there are things of far more import in this world than wearing what is retro, and avoiding what is five minutes ago. While hopefully the viewer finds humor throughout the movie, they also can be challenged to reevaluate who they are, and what they consider life to be. The unexamined life is not worth living- but neither is the unlived life. Does not the best of humor point to something beyond? Does it not call us to be something more than we are? This movie does that.
The writing is excellent, the acting good- especially by Donald Sutherland and Peewee Herman. If you rent it, you'll enjoy it. I'd stake my life on it.