The Back to the Future trilogy is surprisingly cheap on Amazon.
I first watched this trilogy as a kid and it blew me away. I bought it recently, and was somewhat reluctant to view it again for fear that it might not live up to my memory of it. However, it has stood the test of time quite well. The humor is still sharp and funny, and all though some of the special effects are kind of dated, the movies are still just as good now as they were then.
In addition to the movies, the boxset ostensibly contain 10 hours of additional footage. I have not actually seen all of it yet, but the deleted scenes and bloopers are a welcome addition.
Be wary however, the first few batches of the DVD contained cropping errors. This shouldn't be a problem if you're buying it new, but something to watch out for if you buy it used.