Well, cracks are not good. You don't want them to tear obviously. For if they tear, dirts and pebbles can get in, destroying your CV joints in the rear. Now, sure, you can get new shafts, however, from what I was told by a MB mechanic, you would then have to convert to the new "notched" shafts, which then require diff gear modification. ($$$$$$) Now, I'm not sure if my terms are correct, but it goes something along these lines. To shed any light further, my car had cracking/brittle boots for years till they fell apart in the shop while something else was being fixed. I then got replacement boots, but from what I understand, they are hard to find outside a dealership. I've heard of a lot of people speak of re-built shafts, but to fix the car with new parts, its an expensive and involved job. I would trust only a Mercedes mechanic with the right tools....