some personal observation (compiled) after close to 24hrs of use (stock and non-rooted). new items in RED.:
installation:
* need to install via KIES. no OTA yet.
* the update % may slow down at certain times...
* does not wipe your data... but still, never know... can happen. so backup first.
* apps drawer arrangement all gone... have to re-arrange again.

after installation
* cannot disable shutter sound in camera apps
* new swype is nice -- but copy/ select all now requires use of shortcut keys
* seems to take up more RAM -- i know RAM should be used. previously, my RAM usage hovers around 500+... now 600+.
* nothing major. not sure if its just me... but i noticed i get more hang applications today after the update. will probably restart the phone again and see how.
* now got S Memo and S Note... haiz... can't Samsung integrate nicely... now i got two similar apps?!?
* new gmail client -- not so easy to switch between accounts... i guess need to get used to this new interface
* lots of interface change with Google applications, in fact... not just the GMail i mentioned. Reader, etc... all also got some changes.
* Reader Hub still sucks. The ebook reader... still dun have the dark/ night option... previously used to have.
* stock browser seems much faster...
* no new live wallpaper
* i guess i can now uninstall 3G watchdog... ICS has got its own Data Usage meter.
* battery meter can also show % now... yea!
* camera apps... when you launch... you have that annoying TIPS that might not go off... what i did was to try using the S PEN to check "do not show again" and then OK. sometimes might not work. when that happens, use task manager to kill task and try again.
* nice that there's now some guidelines on folders. i see more apps putting photos (within individual folders) into one common folder called Pictures.
* Screencaptures and now called Screenshots. new folders... new locations... new naming convention for the files. Samsung should not change format with each revision la... *sigh*
* battery seems to drain faster. unplugged from wall about 630am. drove to PJ. in between, it was some twitter and reader updates, 5 mins gaming... by about 740am... 90% battery left...
* keyboard hangs... on several occasions, i can't swype anything. had to go to settings and change keyboard before it worked again. not sure if its swype issue or generally the input/ keyboard integration issue.
* TouchWiz actually hung on me about 2-3 times already. basically homescreen will just freeze. can't swipe to other screens... HOME button will light up the screen. i can put phone display to sleep but after unlock (using pattern lock) and HOME screen is showing, it just won't respond. had to restart phone.

seriously, i'm beginning to regret my hastiness with this ICS upgrade. i think i should have held off since this is not OTA and basically that meant, voluntary (and self inflicted)...

major issues i have this this ICS...
* battery drain... unplugged from PC for lunch. phone had 85%. only did like reply to 3-4 IM messages and trying to login to LYN forum. all in, less than 5 mins of 3G data time. i do have like 5 GMail accounts, RSS Reader and Twitter but they dun sync that often. less than 10mins, 3% gone. by the time i got back from lunch about 1 hour later, 66% left. that's like almost 20% gone!

* unstable -- just this morning itself... had to restart phone once and shut down the phone 3-4 times. that's not including the restart during flashing. the phone either just hang (launcher issue?!?) or just got too hot. like just now during lunch... phone temp was hovering at 40C with TaskManager showing 0 tasks running. restart did not work (the one time i mentioned earlier). ended up having to shut down the phone. that 40C was over the one hour when i was having lunch... i tot i'd try to let it cool down but it didn't...

all the above in just my first 24hrs with ICS. i'd NOT recommend this to any casual user like myself (casual as in not into rooting/ modding the phone). i'd honestly prefer GB as i'm used to the quirks and have found workarounds. but in ICS's case... they are quite serious -- to the point i need to shut down the phone, etc...