Echoing a bit what others have said here, but...
Blu-Ray looks and sounds noticeably better than DVD. However, I have an extensive DVD collection and have no interest in rebuying all the titles I collected over the years, so I generally just buy blu-rays of movies that I didn't already own. When I watch DVDs, which get upconverted by the blu-ray player, I am in no way distracted or bothered by their quality.
They still sound and look good to me, just not as good. People were very happy with DVDs for a long time, I think it's a bit of overreaction the way people treat DVDs like garbage nowadays. I imagine when the next super-HD format comes along they'll be saying the same thing about their BRDs.
Anyway, you can get blu ray players for so cheap these days, it's a very minimal investment for the upgrade.