My sister and I swear by Urban Decay Primer Potion, but I'm sorry sis, I may have to stray every once in a while because I found MAC's paint.
Urban Decay Primer Potion is an awesome makeup item that once you rub it on, it disappears into your eyelids and allows for a crease-less and long-lasting application of your eye shadows.
But there are two cons I have encountered with the UDPP. First, it dries clear so that you don't really have any color to serve as a base for your eye shadows. This may be good or bad, depending on your preference. Personally, I like to have my options open, and the UDPP doesn't give me much choice but to go clear, so this is a con for me.
Second, the UDPP's packaging does not allow for you to get to most of the product, as the shape of the bottle is curvy and does not allow the wand to reach every nook and cranny. Unless you take a knife to the bottle and somehow manage to gather all of the product into another airtight container, I might venture to say it could be a waste of money.
An alternative to the UDPP is MAC's paint. MAC carries its own line of eye shadow primer in Light, Medium, and Medium Dark, but I find MAC's paint just as useful. It has a bit of color to it so you can have a color upon which you can base your eye shadows, and it is easy to blend so you shouldn't have too much trouble making it harmonize with the rest of your look.
Also, I noticed that even if your eye shadows are matte, MAC's paint has a sort of shiny quality to it that shows through, and I have seen that this is especially apparent in photographs. And I know that we are all concerned about the creases that may show up on our eyelids, but MAC's paint does address this issue and ensures that crease-less look along with the long-lasting effect, just as the UDPP would. But MAC's paint might also be a better buy because it comes in a tube, and so there is less wasted product.
Decisions, decisions! MAC's paint is probably the better choice, but for now I still keep the UDPP in the back of my eye makeup drawer, for sentimental reasons and for days I would like a clear base for my eye shadows. I'm crazy about MAC's paint, but I still love my UDPP.
MAC's paint is available at www.MacCosmetics.com for $16.50. It may seem expensive for its size: it is smaller than the UDPP container, but I am not willing to do the whole routine with the knife and I feel like I am getting just as much product out of my MAC paint. Below is a picture of my UDPP in XL (available at www.Sephora.com for $22.00), next to my MAC paint in Bamboom.
Hope this helps!