I read about the biochemistry of it here:
http://www.botanic-art.com/ru-21.htm
I think it could have some effect in theory, but I wonder if it lives up to its hype.
It might not prevent "Asian flush" because it supposedly accelerates the acetaldehyde metabolism process, but if you lack the enzyme there's not much to accelerate.
It supposedly slows alcohol processing in the liver in the first place which might slow the onset of asian flush. But I wonder if this could have negative effects in itself.
As for preventing hangovers, a big part of hangovers is dehydration and I don't think RU-21 is going to stop that -- the body is going to pee out the alcohol one way or another. Another part of hangovers is related to substances other than alcohol in some liquors (eg. methanol) and RU-21 doesn't seem to do anything for that either.
Finally, a lot of the ingredients in RU-21 are probably available cheaply in powder form, although I'm not too familiar with what you can buy at health food stores etc.