I finally got my phone yesterday. Compared to the 4, it's honestly not that different. The screen size is much nicer as well as Siri (coming from the 4). My battery seems to be dying a lot quicker than my 4...maybe it's because I've been using it more than normal since it's new. I turned LTE off since I thought maybe that was draining it. iMessage is super fast with LTE.
Also, I got $120 back in activation from chatting with AT&T for 5 minutes online. Overall after 1 day, the phone seems like a nice upgrade, especially if you've had the 4 for a while.