On a blu-ray disc that's well made, the difference is huge. Image quality varies though.
For example, don't watch one of the Tier 5 - Coal versions listed here as your determination on what blu-ray looks like.
"Tier 5 - Coal (Unacceptable)
The titles in this tier have severe limitations in their picture quality that is strongly underwhelming compared to the average Blu-ray. The image has deficiencies in multiple areas that would be obvious upon a casual viewing. Without question the Blu-rays in tier five are among the worst looking on the market. At various times the picture quality is hard to distinguish from dvd material. Softness, artifacting, poor source material, limited resolution and clarity are general characteristics of titles in this tier. "
http://www.avsforum.com/avs-vb/showthread.php?t=858316
EDIT:
This is also assuming that you're close enough to the TV to notice the increased resolution. There are other benefits like better color, but resolution is a bit part of it. If you're going to be a primary factor on what kind of benefit you'll see.
Even with 20/20 vision, you'll have a hard time noticing the difference between 480p and anything higher on a 40" TV if you're sitting 13 feet or more away.
http://s3.carltonbale.com/resolution_chart.html