I went from an iPhone 4 to a 5 didn't want to upgrade when the 4s came out cause I didn't see any good reason to and Siri wasn't good reason for me to ditch the iPhone 4. Now with that said when the iPhone 5 was announced I upgraded. And for me this was a major upgrade. And when I found out that AT&T had just deployed LTE where i live, well let's just say that was a bonus lol.
Upgraded from a 4 also. Without a doubt I am very happy with the upgrade. Specifically for me it is the speed, camera and the increased screen size. An added plus is LTE by the end of the year in my area.