yer them discs some times crack, but its becasue people dont put them though proper heat cycles and expect them to stop them with 100% effecency from 70/80mph straight away. i had drilled discs on my 325i e36 and i went though heat cycling ie, braking lightly from 30mph 2/3 times then a bit harder from same speed, and then a long distance braking break in cycle from 70, say over 1/8th mile.
always bed in my brakes like this and never had problems. and infact find they then brake alot harder then if you just let them bed in nataurally. thing is i hate new brakes but its a must when they are needed to be changed, i do this becasue i hate the thought in the bed in period im going to really need my brakes and not have any response and they glaze up.
if you want cool running brakes just by ebc turbo groove discs, i know there expensive but there worth the cash lay out
