Excellent phone. I foolishly bought a Windows phone - well three actually (the first two crashed after a couple of days!) the other one worked well enough, but it is so nice now to be back with an Android phone, and this is a excellent one! I thought about an S4 but being nearly £200 more thought that a bit excessive, and anyway the rumours are that many of the upgrades on S4 are to be made available for S3 download.
I have only had the phone for a week or so but apart from the very thin backcover - which I half expect to snap in two when ever I remove it - I have no problems with it. I used to like my HTC Sensation but this is superior.
I have been downloading quite a few apps and there is still plenty of memory left,and even if I run out I can add an SD Card up to 64 GB if I choose to. Removable battery also - I know this might seem like a silly point to make - but never very comfortable with Windows phones not having removable battery. It might be biggish compared to many phones but you have got a 4.8" screen - despite that it is easy to handle and feels quite light in the hand.
Screen is great very good colours etc. Range of apps superb including Samsung's own apps.