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 ;)
I only have one iPhone, but I do have iTouch I use to test our app on.