On my iPad3 64gb+3G it works great. I get lots of new features (e.g. Siri) and the 3D maps look pretty awesome in the select areas where it works (e.g. it shows my office and my apartment quite nicely). However the loss of public transport data, the Street view, the Youtube app, etc... is a bit of a downer. The Youtube app (last time I checked at least) was iPhone only (i.e. not iPad size). That being said, it feels a bit snappier and I look forward to Facetime over 3G.
On my iPhone4 32gb however, I don't think it was that great a move to upgrade to iOS6. It feels about the same as before (maybe marginally faster?) but I don't get many of the new features other than Passbook and VIP/Privacy call (e.g. 3D maps, Facetime over 3G, Panorama, etc), but I lose out on the maps details (e.g. Street view). That also being said, I don't feel compelled to buy an iPhone5 which is a bit odd for me- normally I'd jump at the new products but I just don't have any real interest in it (how odd...) Maybe I'll wait for the 5S? *shrugs*
For comparisons sake, I did the iPad upgrade via iTunes (after doing a full backup to both iCloud and my local PC) so ensure it was a clean install. All my apps transferred over great including my firm's sandbox encryption applications for email/document management (which normally requires reactivation whenever I did an iOS update). Pretty happy with this and it's worked well. The iPhone4 however was a bit scary as I did it over the air (using wifi) whilst plugged into a charger. After about an hour and a bit, I had to go so I started the car, and ran to bring the iPhone4 into the car and plug it into the charger. It was about 90% of the way through the first stage of installing (i.e. it had finished downloading). Then whilst driving it had reset the progress bar making me wonder if I had stuffed it up but I remember seeing this on my iPad whereby it has two sets of progress bars to complete. After a while of it staying at 0%, I glanced over and noticed it only had the Apple logo which meant it was booting up!!... and then iOS6 appeared. Everything stayed where it was except Passbook was now on my first page.