My impression so far is great. TouchWiz 4 is so much better then the previous version I had which is 3 I believe. TouchWiz 4.0 feels like a more compelete UI then 3.0 and the visual touches they have made to this version also improves the overall feel of the UI. Of course so far this phone hasn't had any lag what so ever, I thought maybe getting this phone as it came out would be a bad idea, but no unexplained lockups, no unlocking problems nothing, has to be one of the fastest phones out there.
The call quality is on par with a high end device still not a great speaker but you can hear everything and little to no background noise. Taking pictures with this phone is amazing one of the better non Nokia phones I have seen and HD 1080 video comes out clear, the audio is off on a couple recordings but it might just be the way I was holding it or using it, so it could be a retarded camera person (me! lol).
Kiles Air is a great Samsung App that is pre installed at least on the att one, and it makes hooking the phone up to your home network easy, less then a minute. The phone has NFC built in although it doesn't have it enabled yet so when an update comes out it should be working.
A couple things I didn't like, the wireless antenna should be better, when you walk into a store or a bar, the bars seem to drop down to one or two never lost a signal yet or dropped a call, but its only a matter of time, good ole att at its best though. Also within my home (first floor, router on second floor) the wifi drops down to one bar (closer to the router of course the signal went up), not sure if its me or not but I had the same issue the week I had the Infuse so it could be a router signal rather then a issue with the phone, haven't tested it out at Wifi Hotspot yet but will soon and see if it happens. The design is pretty sleek but Samsung uses plastic all over, I have a Zaggs Shield on this phone and did drop it the other day and nothing happened except my heart rate jumped to near death levels.
Overall I have to say I would actually keep this phone for 2 years. The battery life seems to be improved over the Infuse I had but that wasn't hard to do but lasts the day until I get home and still has some life left. Heavy use I got about 9 hours and with lite use the other day I would say it could have kept going for 15-16 hours.