Well, to be honests i'm pretty happy with my S3.
Only cons i can find is the really, really poor (for a 700 euros device) materials quality.
The UI is quite user friendly (in my opinion), well it suffer some lag here and there but (for me) it's really rare (and i'm on no root, stock device).
Plus we will get soon the 4.3 upgrade, and here we will see more improvements for our "baby"... most of all the REAL multicore support, that must "kill" once and for all the lags.
The camera is so good (i'm not a photographer or photo professional user), i find the pictures i snap of good quality and with sammy apps i can modify them however i like. I repeat, i'm not a pro but i love to take a huge number of picture when i'm around with my friends or girl, and my S3 is the best for that job (i don't have to carry around extra weight as other cameras).
There comes now another pro that most of other devices haven't: external SD card.
That's something that any device must have in 2013 becouse of the better cameras (that mean huge dimension for each snapped pic), "consolle like" games (someone is 1-2gb ....) etc. etc....
Now for the "comparison" with a newer phone, my girl have a S4 and i tell you i would never chenge my S3 for it or ever for a HTC one (that anyone is crazy for at the moment)... don't get me wrong, S4 and HTC One are really great devices, but i find useless to drain out your poket of all your moneys, for a slightly better upgrade... Plus with 4.3, S3 will get a lot of S4 features too.
I'll buy a new phone when an amazing Nexus/Galaxy S will be released, but it must be something that will make me really "WOW".