I have had the S3 since march. I have enjoyed every aspect of the phone. my wife prefers it to her iphone. I like the look, the feel and the speed. I was coming from a motorola droid x. what I noticed was it had a slightly bigger screen but was so much lighter. the camera takes decent pics, bringing over apps from amazon and the play store was easy. I will say that if you go with the S3 make sure it is the 16 gig model. Everyone I know who had the 32 gig had problems and the phone was sluggish. I just popped a 16 gig card in and gave myself 32 gig which I don't think I will ever come close to filling.
I have played with the S4 and again it is a nice phone, but not sure if the few new bells and whistles warrant the price difference. If it was me I would go with the S3 with giftcard. Getting a damn good phone for basicly nothing out of pocket is hard to pass up.