I am generally not a big fan of "primer"-type products, usually because they are poorly done or their benefit can be attained from a product you're likely already using. That proved true for some of M.A.C.'s Prep + Prime items, but none of them are ineffective options, and a couple of them are exemplary.
Prep + Prime Skin ($20) is an ultra-light, fragrance-free lotion meant to be applied to skin before foundation. This product doesn't necessarily prime skin better than similar products labeled as moisturizers, gels, or serums, but it does indeed create a silky-smooth, non-greasy surface that can facilitate makeup application. Whether it's for you depends strictly on whether or not your skin needs help in this regard. Compared with other products sold as primers, this formula goes a bit further by being more than a mixture of silicones and water. Prep + Prime Skin contains appreciable amounts of extracts that serve as antioxidants and anti-inflammatory agents. It is a suitable formulation for normal to slightly dry or slightly oily skin, but has one potential drawback: shine. Rather than the subtle radiance achieved from such products as Revlon SkinLights or a foundation such as L'Oreal True Match Super Blendable Makeup, this M.A.C. product's shine is more obvious and more glittery than shimmery. If you don't mind added shine under your makeup (or on bare skin), this won't be an obstacle. All others are encouraged to sample this product with foundation to make sure the amount of shine is not objectionable.
Prep + Prime Lip ($14) is a base that is applied before lipstick to facilitate application and prevent it from feathering into lines around the mouth. Guess what? It works! The silicone- and wax-based stick forms a great barrier to keep color in place, in a way similar to long-discontinued products such as The Body Shop's No Wander and Coty's Stop It! As good as this product is, keep in mind it won't prevent greasy, slippery lip glosses from migrating into lines around the mouth. It works best with moderately creamy or satin matte lipstick formulas, of which M.A.C. has plenty!
Prep + Prime Eye ($15) works well as a cream-to-powder concealer for the eye area as well as for other discolorations. Its creamy texture quickly morphs into a silky, weightless matte finish that is minimally prone to creasing. Prep + Prime Eye can look slightly powdery, but the effect is canceled once eyeshadow is blended over it. The five shades present options for light to dark skin tones, but nothing for someone with porcelain to fair skin. But that's OK, because although this product has its purpose, it is easily replaced by a matte finish concealer, such as M.A.C. Select Cover-Up ($13.50), Maybelline New York Instant Age Rewind Under Eye Concealer ($6.99), or Paula's Select No Slip Concealer ($8.95).
Prep + Prime Lash ($12.50) is meant to smooth and condition lashes, but doesn't do much better in this regard than most mascaras--there is nothing in this product that is all that conditioning for lashes, though it does make them feel soft. It is also supposed to intensify "the build and lengthening quality of all [mascara] formulas," but that didn't happen. Side-by-side testing confirmed what is true of almost all lash primers: they tend to make mascara trickier to apply and don't aid in lengthening or thickening lashes anymore than simply applying two or three coats of regular mascara. I suppose this is an OK option if you're using a lackluster mascara, but the bigger question is why you're not using a superior mascara, given how many (including M.A.C. Zoom Lash) there are!
Source: Paula Begoun