People are getting low scores in QR most likely because they don't study for it too much, thinking that the level of math is easy. It is easy... you just need to know cold all of it, what is VERY important when doing math with limited time availability.

Personally, I haven't spent much time on QR and that is why got only a 19.

Most likely I made a lot of mistakes because I was rushing in order to get it all done in 45 min... not because I didn't know the material.

I used DAT destroyer and didn't find it comprehensive for the math section... I am not sure what are the other math review books for DAT out there but I think it is a good idea to buy as manyas possible. Read through them, and then practice a lot so that for every type of problem that is a possibility for a real DAT you have a solution strategy.