I'm not going to lie to you; you HAVE to be in a good mood the first time you watch this film. I have never come across another film that's as entertaining, creative, nonsensical and just plain silly as this. There is never a dull moment, from the moose business in the credits at the beginning to the totally stupid, but perfect end.
The thing with this film is that everyone has seen it and it's become part of our general culture. Bits of it just crop up everywhere, especially in computer games.
The first time I watched this there were some bits I really loved like the Trojan Rabbit, Robin's minstrels, Sir-Not-Appearing-in-this-Film, the moose thing, the Camelot song, the burn the witch scene, and of course, Terry Gilliam's animations. However, it all really grew on me and I love it now and can quote everything along with the best, and most annoying, of them!
Buy this! It is genius and the actors are brilliant. Long live Monty Python! This film is a national treasure and should be treated as such! Amen.