I've sounded off on this in a few other threads, but I think it bears repeating. :D
WM6 is faster, smoother, better all around experience. Also added full support for 3G allowing for true HSDPA (high speed data) which was not present in WM5.
WM6 also adds Internet Sharing which makes it super simple to tether a laptop or other computer to surf the web using your data connectivity.
A few other little goodies as well like the ability to charge while connected to a standard data-sync USB cable.
HOWEVER, in my opinion, if you're going to upgrade I would load the version for the unbranded Treo. The ATT upgrade does NOT include Windows Live (not really sure why) and they've also locked down the Internet Sharing application such that you cannot tether a computer unless you pay $15-20 more for a 'tethered' data plan. The unbranded version is not crippled in this way.
If you want to load the unbranded update on your treo, go here and ask for the ATT install wizard.
http://www.everythingtreo.com/forum/...isd-13690.html
Oh yeah, you also don't waste the extra space on a bunch of ATT garbage apps that cost to use anyway.
Good luck!