Well i have various hdef movies from a friend, killbill, shrek2, goldeneye etc. I have these all on dvd as well so only have them to compare

. The difference i would say isn't huge but i did notice a slight improvement in detail and its more 3d looking otherwise its the same movie! my dvd's do play very well quality wise so this is my humble opinion! a bit like upgrading a decent graphics card.
Not a huge improvement for the money

don't think about dumping your collection for it, not a big deal really