From what I have read online from many users, the benifits are; more snappy response, less memory needs, and less cluttered OS. much more support for other periferrals for future changes to your system.
I have signed "special" Win 7 Ultimate sitting on my desk, along with the hardware, waiting for the new computers case to arrive so I can finish this install of a new system... while I have a Win7 Pro upgrade from XPpro in UPS route now for the upgrade of another system. So I cannot speak about usability.
BUT as someone metioned about a fresh install....When ever you are moving to a new OS, it is usually best to do it from a clean install. There are a bunch of files that often get orphaned over time, LBA files, install files, and lots of stuff that can use a cleaning. Besides over time your system slowly gets bogged down, and a clean start is often a good way about it. As your system matures with your usage, you can take advantage and arrange things if you think you need to. I HATE doing it, as I have all these tweaks and special setting for brushes, for plugins, for pallets, screen modes in Photoshop, A number automation modes for ACDSee Pro3, for a number of apps. There is likely a good way to transfer these over after a clean install, Would be nice to know where they are. Its time consuming, but I think it is worth half the day...or whole day :-) I have a few systems, and ALWAYS have a secondary main system as work doesn't stop, just because you need a new system. good luck with it.