I'm curious too. I had the 3G running iOS 4 and then I got an iPhone 4 and I gave my iPhone 3G to my mom. She got really excited when I told her a performance update was coming!
It is right up there with 3.1 for me but I'm not sold just yet. I need to play with it for a few days and see how it holds up. 4.0.x worked like a charm for me right after the install and then a few days later just started lagging down.