So far it's mostly all good. Battery life is bad, but we already knew that. I tried several different home screen programs but I'm sticking with TouchWiz for now because the updated one for the Galaxy S3 works as well as anything else I tried, even the backported Jellybean launcher. It has lousy reception inside company HQ but so did the iPhone 4, just an AT&T thing I guess (my Verizon iPad has great reception inside company HQ). I'm still looking for a clean solution for automatically syncing my photos into iPhoto, but iSyncr is doing a reasonably good job of getting them onto my Macbook so I'm not too displeased.
Thus far I've found substitutes for everything I did on my iPhone except one: There is no good offline GPS program like the Magellan program that I used on the iPhone. Supposedly TomTom is going to be remedying that soon. We'll see.
So anyhow, I have found one bug in the Galaxy S3's ICS Android version: It does not handle exFAT very well. I found this out the hard way when my 64GB microSD card quit working and reported, "Damaged SD Card". Indeed, checking the Internet, it appears that random exFAT corruption is an epidemic on the Galaxy S3. This afflicts any microSD over 32GB, since Microsoft officially says FAT32 won't go over 32GB. This is, of course, a lie -- FAT32 is quite capable of handling terabyte-sized filesystems -- but because Microsoft enforces this limit in all their filesystem tools, nobody knew it was a lie until they actually looked at FAT32 and realized hey, this will work with bigger filesystems! (Though there is still that nasty 4GB limit on file size to contend with).
So how did I resolve this problem? First, I put the flash into a Windows 7 laptop and ran chkdisk on it. This found and fixed some problems. But when I put it back into the Galaxy S3 it *still* said "Damaged SD Card" despite the fact that Windows 7 said it was clean. So I resolved to reformat as FAT32. I copied the data off, and then had to go find a tool that would actually format a 64GB microSD card as FAT32 since the Windows disk manager won't do so: EaseUS Partition Master.
At that point it was just a matter of copying the data back on, which was very... very... slow since Windows operates SD cards in sync mode. As in, an hour slow. I know where the async flag lives in Windows and could have flipped it, but it was trash night so I did chores around the house instead. At the end of the process I inserted the microSD into the Sammy and... no more "Damaged SD Card".
Executive summary: If you buy a microSD card with greater than 32GB capacity, it is likely that it is formatted with Microsoft's proprietary exFAT filesystem and will not work well Android unless you reformat it, even if it appears to work correctly at first. exFAT support is not supported well because it is patented by Microsoft and thus does not have the magic of dozens of eyes of Open Source developers noticing and fixing bugs in it. So reformat it using the EaseUS tool above (NOT the internal Samsung formatter, it'll put the buggy exFAT filesystem back onto it) *before* you put stuff on it. Otherwise you'll be going through this whole time-consuming dance yourself sooner or later. Fun, it was not.