DVD commentaries are a fascinating insight into movies and one of the most vital ingredients for purchasing a DVD, IMO of course!
I love watching a film and then being able to compare what I felt about the film with the intentions of the actors/directors/writers
Small stories, little notes, explanation of possible alternative scenes...... if you like films or film making they are an excellent resource (a point mentioned on the American Beauty commentary)
I remember the first commentary I saw was for The Usual Suspects and it came on a separate video to the film! Clunky!
I've noticed a decline in DVD commentaries at the moment which is very irritating. Image stills, Trailers and the like you can grab off the net and interviews are regularly on TV as part of the films promotion - but commentaries - priceless!
Top 5 commentaries I've seen:
1. Fight Club: Interesting and humorous. Brad Pitt comes over as quite a character (I was quite surprised!)
2. Usual Suspects: Fascinating and witty - I like the way Singer can laugh at his own mistakes ("The plane goes into the air with four engines...... and oops comes down with two!.. oh well....")
3. Thomas Crown Affair - Excellent if you are interested in Film making.
4. Gladiator - Same reasons as above.
5. Ghostbusters - Funny, intelligent and you get tiny heads at the bottom like in MST3K!
Pity X-Men didn't have one in the UK, nor did American Psycho - now that's film that would have benefited a commentary!