It's really great for the skin alone! Makes your skin look fresh, dewey and bright. With foundation, of course makes your foundation (even the cheapest ones!) last longer. Even just plain powder on top if you're a bit oily.
The second one and the one I usually use when the weather is really weird. When it's sunny then in a few hours it's going to rain!
MAC Prep + Prime really does it's job. Although one thing I don't like about it is its consistency, it's a bit runny, the product and the liquid is separated but it's a great product.