I upgraded from the 4 to the 5 about a month ago and I couldn't be happier.
The 5 is MUCH faster. I know in all reality we are really only talking about seconds or fractions of seconds, but launching and moving between apps on the 5 is a much more pleasent and seemless experience. It is especially nice if you are someone like me who is on their phone constantly, moving between apps constantly.
And of course the added screen size is a plus. It's big enough that it offers added, useful space, but in a way it really isn't that noticeable after using it for a while. It's obviously noticeable upon first use, but you adjust to it quickly. It's still a "comfortable" screen (as opposed to something like the Galaxy S3...it's a big, beautiful screen, but it is almost too big for a phone IMO). Hard to explain. For me, it is really nice having a longer screen when using the Facebook app...it makes the newsfeed feel less congested, especially now since they have worked mobile ads into the newsfeed.
More importantly about the screen, I was really pleasantly surprised about the upgraded screen quality. Immediately I noticed that the darks seemed darker and colors just "popped" off the screen more in general. Going from one Retina display to another I wasn't really expecting any quality bump, but this was a very welcomed surprise!
Also, the bump from 3g to 4g is great! My data speeds have more than tripled. My 4g data connection on my iPhone is now faster than my wifi at home, lol.
And of course the upgraded camera is awesome. I take pictures with my phone all the time, so any improvement on the camera is great for me.
And unlike many others, I actually prefer the form factor of the 5 over the 4. I was really skepitcal at first, and just looking at the pictures of the slate backing, I really wasn't a fan. I liked the all glass back of the 4 as opposed to the (what appeared to be 2-toned) slate and glass back of the 5. After handling it in person I changed my mind quickly. The slate offers a matter type feel, which feels great in your hand and offers better grip. I also really like the flat black ring/trim as opposed to the shiny silver ring/trim around the 4. It looks like a much more sleek device overall, IMO.
Edit: Oh yeah, my battery seems to last about 15-20% longer as well.