I've long since held this opinion, but I used every jailbreak trick in the book to make my 3G run the full iOS 4 experience and it failed miserably. I activated home screen wallpapers and multi-tasking, but had to delete the text and icon drop shadow to make the respiring bearable, and even then it wasn't much so.
Multi-tasking worked great as long as you stuck to keeping stock apps and lighter apps (Facebook, Twitter, etc) in there. Leave TomTom or a large game running in the background and things got even worse performance wise.
My Father in Law got a 3G two years ago (his contract expires in November) on O2 and I only recently jailbroke it for him on 4.3, and it brought back all those bad memories, only it felt even worse being used to the iPhone 4. I made his iPhone far more usable than I ever managed to make mine by installing FakeClockUp and setting it to x10 so you don't see the animations on respring at all ... and he loves it, lol.