I installed 4.0 a few days ago on my 3G and only notable difference is the folders. I think to use multi-tasking I need to upgrade my phone to 3gs? Stinks if that's correct!
Did you install 4.0 on a device you use regularly? If so, are there not many bugs hindering your usage? I've been thinking about doing this as well but Apple recommends against it.
Of course, no apps background properly currently, but it's useful for testing.
I'm using it on my primary iPhone, but I carry a backup phone in case I actually need to get stuff done ;)