For what its worth, my views on the device... (After using a vanilla Amoled HTC Desire for two years)
Aesthetics: Listen it's no One X, lets leave it there...but as said by many before, looks much better in hand than those internet pics.
Build Quality: I REALLY think Samsung could do much better here. Plastic, do I say more...
Size: Little big for one handed use if you have small hands (Nothing else correlated to the hands

). Absolutely no pocket-ability issues, don't even notice the device.
Weight: Feels very very light in-hand and in-pocket. Probably due to the bigger screen real estate.
Display: Although the One X apparently blows the S3 out of the water, the PenTile display on the S3 is really not that crap, actually very nice. So to everyone on world wide forums and review sites bashing the display... please get with the F.... program. This was almost a deal breaker for me after I read how crap the display was in comparison to the One X.
Performance: Well with all the crap that's on the phone including the initial TouchWizz or whatever they call it (Launcher changed immediately) the phone is very snappy. But it is a Quad Core 1.4 after all so anything other than snappy would be a great disappointment.
Battery Life: INSANE INSANE INSANE, maybe I'm just used to the Desire but this thing is amazing. On my first charge I got about one full day and that was with an insane 6 hours screen-on time, Display Brightness set to just under half way...nice and bright, lots of fiddling around, some Whatsapp and plenty TapaTalk. WiFi and syncing on the whole time. (2 Gmail accounts)
On a more technical note, while sleeping yesterday and last night the maximum current drawn was -4mA resulting to not a single % point drop in 8 hours sleep.{Do have the screenshots to prove it, really not gonna upload them, just have to take my word}
Currently "Cell Standby" seems a little high, have read on XDA that there might be an issue in that regard.
Don't even wanna guess how it will perform in a months time.
(Side note on battery use for Android newcomers. Search the forums, Elimentals and many others have posted valuable info)
{Bit OT, but I gave wifey the Desire. I'm a HUGE AOSP fan and ONLY used Oxygen by AdamG on the Desire. Did a nice full wipe for her and her current Battery stats as follows: 73% Battery Left, Time since last charge 23h48m Screen-On 57m49s That is with WiFi on the whole time, occasional Syncing, Screen Brightness Auto, Some emails and whatsapp. For a Two year old battery I don't think that's to shabby}
Upgrade advice: Bring the flaming on... Coming from the Desire and after using a Galaxy S I9000 today, I see absolutely NO reason why any S2 users should upgrade, unless obviously for some specific reason... bigger screen or whatever rock your boat. (Screen size is actually the only thing I can think of.) {I've never really used the S2 but I think the 4.3inch might be a hit and many "normal" users might find the S3 display to be a little overkill.}
After running AOSP on the Desire I still don't see such a huge speed difference between the two devices. I'm not a gamer, so please see this review in the light that it's been written.
Cheers