Pros:
Bigger screen
Flexibility & Customization
More free apps
Open source
Faster processor
More RAM
Bigger battery - replaceable with lots of choices depending on need
Did I mention bigger screen!
Great looking device
Expandability with SD card option
Color choices!!! (New)
Cons:
Camera not as good, but more options on GS3
Customer support - no Apple Care or Genius Bar. This IS A BIG DEAL if you are the type that needs to talk to an actual intelligent support person. If you can get by on forums like this or dealing with the gurus at the carrier then GS3 is fine
Bluetooth - GS3 cannot connect to 2 headsets. If AUX cable is plugged in BT headset won't work. S-Voice doesn't connect to BT headset mic.
USB - cannot connect to USB hub in vehicles to stream music and control music with the vehicle's stereo controls and song / artist info does not display since it won't stream via USB.
No iTunes Sync - Must create alternative work arounds for music/photo syncing (no iTunes auto sync) . There are several good work arounds though. I use iSyncr and it works great!
There is a learning curve coming to Android. For me the curve was all the flexibility that was at my finger tips. It really is cool how much you can tinker and make the phone your own versus a very sterile UI.
I tried Android a little over a year ago and went back to an iPhone when the 4S came out. I had the Droid Charge and it was very troublesome. When I saw the S3 I felt like the time was right to come back and I am very glad I did. I have had to make a few adjustments, like BT and iTunes sync, but I have found solutions for each item. The help on here is great too!
FYI - I am still very much an Apple person. iPad3, wife and daughter both have iPhones, iMac, Macbook Pro, Macbook Air, Apple TV and Apple router. At first I felt like I was cheating by buying, then actually liking the GS3, now I think that diversity is good. I even bought a new PC Ultrabook to add to my collection of tech and passed my Macbook Air to my (spoiled) daughter.