Well, I did it, not without some hassle though. On my XP PC it got stuck 1/3 of the way through the iOS4 install, and got stuck at the same point of the restore, three times. I ended up connecting it to my flatmates Mac, he did the restore which put iOS4 onto it, and when I connected it to my PC again it put all my apps and data back onto it. Without the Mac i'd have been stuffed.
So far I like it. It's not significantly slower, I like the enhancements like app folders, email changes, and the general tweaks. One thing i've noticed is when I turn it on it has the wireless network already, which suggests it's not letting it go, which might be why people are saying it uses more battery. I'll probably just turn wireless off when i'm not using it if I find it's a problem.
I'll report back if I have more impressions.