iOS4 has some pretty neat features, but many existing iPhone 3G users have found performance to be pretty poor after making the switch. Before rolling back, try a couple of hard resets to see if that improves performance.
Lifehacker reader Tom was suffering from the iOS4 blues, but ran into a handy fix via the Cult Of Mac site:
It appears that two hard resets (press and hold both buttons, ignore the "swipe to turn off" prompt, then release the buttons once it restarts itself) somehow clears and purges a bunch of memory. This gives applications more to play with, meaning they page the hard disk less. Since paging the hard disk is slow, this ends up making the 3G seem a lot more zippy.
While this may not work for everyone, it's certainly worth a try if you are seeing iOS4 performance problems. If that doesn't help, you can roll back to the old version. Thanks Tom!
Speed Up iPhone 3G + iOS4 with a Hard Reset? [Cult Of Mac]