You can't tell a huge difference until you watch a couple of Bluray movies and then go back to DVD. You'll literally wish you never bought all of your DVDs the quality is so much better. Good reason for Netflix
One thing to keep in mind is that many broadcasted over the air channels (ABC, CBS, NBC, Fox, etc.) are actually in 1080i. This is very, very, very close to the quality of Bluray movies (1080p). So if you've seen a high-def football game on ABC, you've seen Bluray quality. It's a good way to see how much better the quality is for free