If you have a first-generation iPhone 4 (i.e. running the A4 processor) and are considering upgrading to iOS 7 my recommendation would be to hold off until you have a new phone with the processing power to handle it.
After upgrading to iOS 7 after a few weeks ago, I can safely say that while the phone remains usable, it is a lot more sluggish even for basic operations. Sometimes the lag is barely noticeable, sometimes maddenly so.
It wasn’t too hard to get used to the changes in the interface, but I while the UI for iOS 7 is different, I’m not sure it is any better. Learning the new way of doing things felt like climbing a hill to get back to where I was. Except that everything is a bit slower.
So aside from getting to play with the new UI, in my opinion there is no upside to upgrading a first-gen iPhone 4 to iOS 7.
Update (Mar 10): This Ars Technica article takes a more detailed look at running iOS7 on an iPhone 4 and more or less jives with what I’ve experienced myself.