I sold my previous iPhone 4 and got an S3 a couple of weeks ago, but I'm planning to switch to iPhone 4S again.
The S3 is a great phone overall, and is quicker than my previous iPhone 4. Android is a good OS, though not quite as slick as iOS in my opinion.
The problem with the S3 is the screen though. On the one hand, it is a nice size, making it very usable. On the other, you need 2 hands to operate it most of the time. White backgrounds seem a bit washed out and nowhere near as sharp as an iPhone 4/4S.
The main problem though, and the reason I'm switching back is the fact that you really can't use the phone outdoors if there's even a hint of sun about.
Even with the brightness turned upto the max setting, you can't make hardly anything out. This makes trying to use the very good camera almost impossible for outdoors shots. Really, I'm suprised that there's not more critisism for what I see as a serious design flaw on the S3.