The metro apps, specifically Music, Mail, and Photos are definitely improved! I'm giving the Music app a kick around and it's pretty decent, it's more like a Zune-lite than anything, but VASTLY better than before!
Installation of the Release Preview has got to be the most smoothest installation of Windows I've ever done in my life. Period. It installed fast, and everything literally worked right off the bat, as usual. I guess at the DP stage, I signed on with a Microsoft account, and I signed into it with the RP, and everything synced over, it was shocking. My facebook, my Calendar, my email, and even my Desktop wallpaper and window color synced over!
Metro IE 10 has been improved, you click on the URL bar and your favorites appear ABOVE it, not like the black screen it was before with the CP.
Pulling up the Charms bar feels much faster than before.
It is definitely faster than the CP! Everything just feels like hot oil, it's so fluid and fast, WOW!
There are much more colorization options in the initial setup, where it gives you little tacks of tiles with the screen color and accent color, my concerns about personalization of the Start Screen has dissipated some.
Live tiles on some of the apps are better improved.
The Desktop remains fairly the same as the CP. Microsoft wasn't kidding around when they said the RTM will literally be the final build with the final changes! There are some cosmetic changes, such as the mouse pointer doesn't have a drop shadow, there are a couple new pointers to replace the Aero "working" mouse pointer. The Ribbon is flat, and looks actually decent, but it's not final of course. Some of the old Aero UI bits, like the progress bar, is a solid green now. But you can tell the Aero in Windows 8 is like a defunct ship now...
Overall though, it's mainly the Start Screen and the metro UI that have seen good refinements to them, but some work is still needed. The Desktop isn't final in this one, but there are cosmetic changes that definitely show change. Windows 8 is shaping out to be very different and a far cry from the Developer Preview. The metro aspect works greatly better than the CP, though there are some bugs to be worked out again.