I believe that it works - at keeping the back of the bus clean.
I actually saw a bunch of buses on the highway with those on them about 2 months ago - in Maine. (Now I know what they are.) I can see how on dirt roads, or in states that sand and salt the roads, getting rid of the giant cloud of spray, salt, muck, and water behind the bus would be a good thing, both for the bus driver, and other cars on the road.
As for any gain in MPG. . . maybe, maybe not. Any turbulence creates drag - weather that's turbulence at the front, OR the back. It should help, but I don't know that it will help enough to really be statistically provable - which you have to be able to do to market it as an MPG-improving tool.
