My favorite new feature seems to be the panoramic camera. I have a 4S, so only 4S and 5 get the panoramic.
Also the phone is faster. Safari loads much quicker, LOVE the full screen safari if you tilt phone sideways... really enriches the Safari experience even on a 3.5" screen. The 5 will REALLY be able to take advantage of this with the bigger screen.
Also iMessage. My iMessages send instantly now. Before there would be usually a 5 or more second wait before it sent. Love this, since most of my texts are iMessages.
Again, I think iOS6 has improved my 4S. My battery even seems to hold a full charge for a longer period of time, but I haven't had a chance to really test it out yet.
I suppose if you have a 4, the update isnt AS GOOD because you don't get all the features like the 4S gets.
But in my opinion, the upgrade is nice.