Re: UDPP vs. MAC Eye Prep & Prime
I don't consider them to be the same type of product, even though they both have primer in their name.
In my experience, MAC's PP line just makes application easier and smoother. It doesn't do anything to increase wear (eye or face) - but it does make a difference in texture, especially under foundation.
UDPP is similar in that application is easy (provided you just use a little - instead of globbing it on), but it brightens colours, and keeps those shadows and pigments in their place.
I have both, but I probably wouldn't buy MAC's PP again, not because it doesn't work (it does), but because I generally don't have a problem with uneven eye applications. If you're looking for anti-crease and holding power, I'd definitely recommend UDPP.