Destroyed my BB before 3 months was up..
The cranks arms themselves aren’t much lighter than the 07 Gravity arms. The weight shaving is primarily the bb.
I have 08 Gravity lights, and have already killed both sides' bearings in the BB, the cranks themselves aren't damaged, and I'm not an abusive rider, I just believe they're weak BB's. I cannot speak for how tough the cranks themselves are, but when the BB dies after a non-abusive 3 months I have very little confidence hitting up big air. Imagine landing, having the crank snap, and have it get nice and intimate with your calf muscle..NOT COOL!!
As for the current Saints, I've read that the extra weight doesn't = extra strength, other brands have much better strength/weight ratios. I have also heard that Shimano have addressed this for their new Saint set, weights down within a competitive range and stiffness and strength are dramatically up...
I run Race Face evolve cranks and BB on my other bike. Flawless. Not the lightest, but definately alot stronger than the FSA's and strength/weight ratio is said to be higher than current saints too..
So after all that, if you didn't read above, don't get Gravity lights if you're doing downhill, look for older Saints or Race Face's. Both are heavy, but are also alot stronger and more confidence inspiring than the FSA's.