what color car?
I had a clear bra on my Boxster (after I had it's nose repainted) and it did a phenomenal job keeping it rock chip free. However, it was Arena Red and I used the InvisibleMask.com brand not 3M. Here are the reasons:
1 - I had the 3M put on my wife's white car and yes, if not properly cared for every 3-4 months, it will start to yellow. Obviously notable on a white pearl metallic car but easily correctable w/ a good cleaning and polish
2 - The 3M product has more 'orange peel' affect in it than other brands and they use a yellowish adhesive. That being said, 3M apparently has reworked both a bit. A buddy just had is new white car done w/ 3M and truth be told, the adhesion agent is not nearly as yellow as it has been in the past. Nor is the orange peel as noticeable...
3 - InvisibleMask at the time had a product that was a little bit thicker than 3Ms offering. Not sure what they are today
So, IMNSHO, it depends a lot on what color car and being able to find a GREAT installer (not just a decent or good one) to make a difference. If you can't find a great installer, then honestly I would just get it resprayed every few years. A sloppy install would make one rather have rock chips...
Good luck
