I haven't tried any of MAC's mascaras because I am so addicted to DiorShow, but I did just purchase Prep + Prime Lash. I can see a major difference in length, but it does make my lashes appear much drier and a little off color (more of a greyish than deep black). The lengthing results are enough to make me keep using the product.
I have the Stila primer and for the price, it's nice. Not great, but nice. I prefer Clinique's formula, but I picked up the Stila one for $6 at my CCO, so can't complain!