This one is a real entertainer. This movie is genuinely funny and genuinely a mystery. Hrishkesh Mukherjee was a genuinely good director and story-teller; something which a lot of filmmakers seem to forget nowadays. You should also watch some of his other films: they are very different from each other and most are in the lower-budget range but with excellent results. Music and songs are also memorable.
Just adding the comment to Yellow_Spider that his comments for this film were being lifted "word-for word" by the presenter of "Bollywood Ke Sunehre Pal" of smashits.com. Watch the November 17th 2006 episode's introduction to Buddha Mil Gaya and the song "raat kali ikk khwab me ayee" Maybe you should be acting as their writer.