In my experience, the iPhone 4 is pretty great. Relative to my 3G, which was on the verge of collapse after two years of software updates, undone jailbreaks and loads of intensive apps, it's way more stable. Not being on AT&T's California network is a refreshing reminder of what cell phone service can actually be like. Everything is way faster, which makes me appreciate how fast the network can actually be when things like Facebook updates load instantly. (Who knew that was a hardware limitation?) TomTom loads and operates quickly and navigates more accurately.
Games are great with the new processor and screen. Between Nike+ and another new Nike app, I'm back to working out with my music. The new glass is more precise and less smudgy. And I haven't taken advantage of the improved camera much yet, but I'm excited for the first time I'll snap a quick picture and think "I'm glad that got updated!"