I haven't had background installed since I've been on before 4.0. Any app the isn't updated for 4.0 will not even show up in multifl0w. It does not close apps that save state or 'multi-task' nativity under 4.0.
I just tried with some more apps, mine closes literally everything with the exception of apps that I have set to "overide: continue to run app when closed" in backgrounder.
If I'm in the photo albulm looking at images, I can multitask around and come back to photo and its still on the image. When I close it in multiflow, it has to reload the whole app, I have to tap the camera roll etc.
It completely closes every game I have tried to close, ios4 compatible or not.
It does crash my springboard whenever it shows more than 6 apps and I try to move them. (which I only did to **** with it)
Other than the infrequent crashes, its been great for me.
Like I said in an earlier post, double tapping home on the lockscreen still brings up iPod controls.
And I don't know when they fixed it but in multiflow if I tape the space around the apps it takes me to the springboard homepage. Pretty cool.