Holy crap! I love this phone! I've been using it for about a month now, and it's absolutely light years ahead of my experience with the previous generation Galaxy S (Fascinate). I can't keep my hands off the phone... it's such a pleasure to hold and use that I find myself looking for excuses to use it. i.e. I do things like read emails on the phone even when I have my laptop open to email right in front of me.
TouchWiz:
I've been using Launcher Pro on top of TouchWiz and am very happy with it, the two launchers actually combine quite nicely. TW actually adds some value with the toggle buttons in the notification bar, and I actually really like the TW Browser.. it's fast responsive and generally pretty well put together.
GPS:
There doesn't seem to be any sign of the previous generation's issues with GPS. Lock is fast and accurate.
AT&T:
I was pretty apprehensive about making the switch to AT&T from Verizon, but I'm actually pretty happy with the change. In the Washington Metro area I haven't experienced any dropped calls yet, although I have experienced a few occasions while driving where I could hear the signal start to break up. But overall the voice service of AT&T has been just fine here, and in California. In terms of Data I'm very very pleased, HSPA+ is WAY faster than Verizon's 3G, and while it's not as fast as Verizon's LTE it's plenty fast enough for almost everything I've wanted to do so far. The exception will probably be video chat, because of HSPA+'s upload speeds not being stellar all the time.
With Speedtest.net the best speeds I've tested were 6.11Mbps/2.00Mbps and the worst I've seen was 1.56Mbps/0.54Mbps. Average seems to be in the 2.5-3Mbps/.5-1Mbps range.
I didn't think I'd care about the ability to talk and browse at the same time... but actually it's really useful. Browsing during boring con-calls, or having access to data for navigation while on a call is great.
Battery Life:
WAAAAY better than my Fascinate! I'm a very heavy user of my phone... I typically spend at least 3 hours a day on calls... lots of browsing, gaming, email etc etc. It's usually about 14-16 Hours between when I unplug til when I plug back in at the end of the day. Typically I have about 25-30% battery remaining. For me, thats extremely good.
There does seem to be a weird issue going on where I see Android OS accounting for 18-20% of battery usage. This seems to be some kind of issue, and there's lots of discussion about it over at XDA. This is something Samsung really needs to fix, because there's no way that process should be leaching so much out of the battery. Nonetheless even with this weird leak, I'm very very happy with the battery life.
Accessories:
I ended up buying 3 different cases... the first was a generic velvet-ish pocket. I used this initially to protect the phone while waiting for the other two cases I ordered to arrive. Obviously this isn't a great option... but it worked ok as a stop gap.
I also got the PDAir 3BSSGXVX1
http://www.pdair.com/catalog/product_info.php?cPath=10100000_10000000_11001206&products_id=32476
and
the Case-mate BarelyThere
http://www.case-mate.com/Samsung-Ga...te-Samsung-Galaxy-S-II-Barely-There-Cases.asp
At first I really liked the feel of the Casemate, and was using that for the first 3 weeks, but it quickly became apparent that it stole a good bit of the beauty of the phone and it didn't do a great job protecting the top and bottom edges of the phone. So in the last week or so I switched over to the PDair. Love it so far, I get to show the phone off in it's naked glory, and still feel comfortable keeping it in my pocket next to change and keys. I am terrified that I'm going to drop the thing while it's outside of the case.
I've got a Skinomi Techskin on the screen, I much prefer the Skinomi's wet application. It's easier to get everything on perfectly, and unlike Zagg it doesn't feel like rubber to the touch. It gets a little greasy from my fingers by the end of the day, but it does self heal well, and it protects the screen very well. Other negatives of the skinomi, it collects some lint at the edges of the protector... but I think thats a universal problem with screen protectors.
Bluetooth... I picked up the Jawbone Era... which works perfectly with the phone. The Jawbone companion app works perfectly as well, even though the SGSII is not on the list of compatible devices.
Update Conclusion:
LOVE this phone... by far the best phone I've ever owned.