The difference isnt as big as VCR to DVD, I have a few blu-ray movies, but I'm not blown away to where I'm going to replace my DVD's to the new Hi-def DVD's. I thought I would, but my regular DVD's have a great picture.
Transformers in HD-DVD is enough to want me to get all my movies in HD. It's not just the clarity of the picture, the colors just look better. And there's nothing like actually having the special features in HD.