I would get an iPhone 5 with a contract soon, but I'm moving to Singapore in a year, so two-year contracts aren't even an option. I'll tough it out with my 3GS for another year, and get a 5 in Singapore later. I'm a pretty big Apple fan, their products are quality, although pricey, but if it's in your budget, I definitely recommend what they have to offer.
Using iPhone 4 here and haven't upgraded to iPhone 5 yet, but I have tried my friend's and its really much faster. Personally I would upgrade to iPhone 5 but it would be wiser to wait for the prices to drop first and maybe get a second hand one since the devices don't spell easy anyway and its just much cheaper that way.
I'm not a fan of apple, and even less of the Iphone. In my opinion, they ripoff people when creating a new iphone with some minor new functionalities/upgrades and pricing it that high.
I got an iPhone 4S and I love it. The iPhone 5 has too less improvement imo. I hope that the next iPhone will have more improvement...