So far I've updated two phones. Both updates went smooth. Only one app that put the date on the status bar doesn't work which frankly doesn't matter now as Gingerbread has a nicer display on the unlock screen anyway.
I've noticed the phone is definitely more responsive. Before updating I was comparing the two Epic's we have and it's just a bunch of little things like how fast if goes from portrait to landscape, or how fast an app opens, or how fast Bluetooth connects or how fast google voice search responds or the fact that Skype is actually useable on Gingerbread where is was very poor/useless on Froyo.
I have also noticed the battery life is better now. I haven't had a chance to do a proper test, but as I have a 3500 extended battery, the change is probably more noticeable. Even with the extended battery if I turned on Wifi and Bluetooth I would see a few % drop after a phone call or checking/reading mail. Normally as of this writing I would be down to 80-85% by this time in the day WITHOUT wifi on. Today WITH wifi I'm only down to 90%.
I am a power user, but not a super user. I use the phone, bluetooth, wifi, email, texts and internet and get over 200 emails per day. I don't play games except for the occasional Solitaire or Angry Birds.
Other users may have different results, but so far for me... the phone is finally acting as it probably should have been acting, when it was new. No complaints.
I'll keep testing and do a good battery test as well. I'll do some testing on the stock battery as well. For us the stock battery was pretty much useless. We had to keep the phone on a charge, especially using the GPS. It think it may be different now, but that's speculation.