I notice a pretty large difference on my 52" XBR4. The only DVDs I still watch are the Lord of the Rings Extended Editions (aside from when my nieces and nephews areover), and they're extremely soft and fuzzy compared to the Blu-Rays I own (except for those with shitty transfers, like Talladega Nights, which I got for free with my launch ps3).
Even an older film like the Fifth Element looks much more detailed on Blu-Ray than any DVDs I've seen.
I really can't wait for the Lord of the Rings on Blu-Ray!