Many people say they don't see a difference in quality.
To each his/her own. Also, they WAY your rig is set up can make a big difference.
The bigger the screen, the bigger the difference. Also make sure the video settings are adequate; you can have a $19,000 TV but if it's not properly adjusted, shit will still look like crap.
With that said, I would recommend watching a movie that you've seen several times on DVD, then get the blu ray based on reviews from blu-ray.com. They are reliable.
A big difference to me was BLADE RUNNER. It's fantastic. But there are hundreds of movies now that blow the DVD format out of the water.
Some aren't great, but most are at least a noticeable improvement, if not a drastic one.