I just tried it, and it does what it says it will do.
I agree with the people suggesting that it would be helpful if there were control over which pages or categories the background appears on. At the moment I'm using mostly Pages, which don't have Categories nor Tags.
I happen to be using an iframe for one effect...resulting in two different backgrounds being visible and not at all the effect I'm after. I also have several different types of pages within one site, so it would be nice to specify a different NEXTGEN gallery for several pages.