Leo's been using iPhone 5 for a day and hasn't really noticed it being really appreciably faster than his Galaxy SIII. There's two questions about the iPhone 5. The significant difference is that it has one extra icon row, or 176 pixels more. If you're in an LTE area, it may be worth it because of the fast connectivity. Running a speed test, Leo got 27mbps down and 15mbps up. That's faster than home internet, but that means you'll burn through your data caps faster too.
As for the screen, it's not all that big. If it's a big screen you want, there are better choices out there. The Samsung Galaxy SIII has a larger screen. In fact, Leo says the best thing about the iPhone is that it improves the choices out there. It continues the belief of Leo's that you can choose a carrier first, and then select the phone you like best.
Another issue is the connector. It's caused Leo to get rid of a lot of his 3rd party docks and connectors. Even with the adapter, iPod control will not be available through it. Also, there's a shortage of cables and he has only one cable to charge his phone
The bottom line is, if you're overdue for an upgrade from the iPhone 3GS or iPhone 4, then definitely get it. If you have an iPhone 4S, then it may not be worth it.