I had Goldwing do it for my X5 (E70). They did not get it right at first and so redid it no questions asked (It was an early E70). I had to look very carefully to see the clear bra. After 3 years there was no discernible fading or yellowing. I was satisfied after that. I went back for tint and detailing of my E91.
Since moving to Europe I have not seen clear bras on any cars so I can't comment on how they look on an M3.
Collelongo (AQ) BMW Club
Sometimes I wonder why I drive BMW's then I drive something else and the doubt disappears.