I haven't upgraded yet, though I am sure I will soon. The problem I have with Itunes is it is hard to find past versions. So if I don't like the new version for some reason, I will have to do some searching to find an old installer.
A lot of the feedback people have been providing is positive. Describing it as simple and sleek (as though it wasn't). From what I can tell, no major changes to the user experience, just some cosmetic stuff.
Check it out for yourself. http://www.apple.com/itunes/