Actually, it seems to depend on your software a lot more than hardware. iPhone 4 users (like me) seem to get a speed-up if they regularly manually close background apps and tweak the settings to block uncessary GPS use, and a slow down if they don't.
My phone *seems* snappier at some things, but probably isn't. For example the unlock screen is noticeably more responsive, but that's probably because of the much bigger swipeable area, and the 'swipe a notification to launch the relevant app feature. The only noticeable slow down is the zoom animation when launching of leaving an app, but that's more thanoffset by the things that take a lot less taps than before, like turning off wifi.
The best feature for me is one that hardly gets mentioned in reviews; the ability to block contacts. No more 7:30 am text messages from the dumbass phone company telling me my bill is ready to view, and that idiot who can rarely dial his daughter's number correctly will finally be silenced. Considering the phone company wanted me to pay them half my line rental again to do this, getting it for free is a great deal.