as far as I know, there used to be major problems with some red hat rpms in the past, so I am kind of prejudiced against it. besides, mandrake is free, unless you want star office, while you have to pay for red hat, as far as I know.
I think mandrake is less user friendly than red hat/suse, but personally I had not many problems with mandrake, although you will need some time to learn how it works, but then again, no linux distribution is as user friendly as windows, so this is not a real disadvantage of mandrake.
apart from that, as ipdave already stated, mandrake is just a deviate of red hat, so there are just minor differences.
if you want mandrake, wait till v.9 final comes out, maybe kde 3 is included, kde 2.2 sucks.