Ive recently changed to Android after having an iphone for around 3 years, i had the 3GS and 4S.
I'm not regretting it one bit, the Galaxy S3 seems to be a great phone, its lightening fast and the screen is amazing really.
The things i like about Android and the Galaxy S3 are as follows, apart from it being lightening fast with a great screen i like how customisable the phone is. With the iphone your stuck with the memory you buy, with the S3 its great how you can add a micro SD card upto 64GB and thats on top of the internal memory that the phones comes with.
You can set your own email and sms tones as well as ringtones, with an iphone it has a sealed back so you can't get inside the phone easily if you need too, with the Galaxy S3 the back cover pops off really easily allowing you to change the battery if you want too, i carry a spare around which isn't an option with an iphone.
The android app market is now a lot bigger than when i used to remember it when i had a HTC Desire, its great and the apps are a doddle to install. Their is much to the phone and android, ive just skimmed over the surface really.
I'm more than happy making the move back to android and the Galaxy S3 seems to be a great phone so far.