I should note for the record that I am one of those girls who has a horrid time with eye makeup's durability. While my skin is largely dry, my lids tend to be a bit oily, and eyeshadow tends to just slide right off. Eyeliner tends to smudge and smear on me, no matter how good it is (the only one that didn't budge before today was MAC's Fluidline).
So, it's an accomplishment for me to be able to say that I sit here writing 11 hours after putting on my makeup, and it's still exactly where I applied it and looking just as good as it did this morning. Today is the first time in my life I can say that. Not at all coincidentally, today is the first time I used UD's Primer Potion. I applied it both on the lid to the browbone, and in a thin line under the bottom lashline where I use eyeliner - and I cannot believe what a difference it's made. It's rare that I look at a makeup item I paid $14 for and think to myself, "I'd pay double for this without blinking." Liners apply smoothly and with high pigmentation over PP, as do shadows. The colors stay true and vivid. It's utterly amazing how much of the actual *color* shows over PP. It's also amazing that none of it sinks into my somewhat-deep crease (as is my norm) or makes me look like I have stripes of color on my lids. It's smooth like buttah, girls.
My only problem occurred when I tried to blend - the PP made blending eyeshadows a bit more of a production than usual. This is easily alleviated by learning techniques to work with it, and with practice, I'm sure this problem will disappear. It just took a bit more brushwork to make the colors blend smoothly. I don't consider this a shortcoming of the product, however; this is merely proof that it does exactly what it's supposed to do, from the moment you first apply makeup over it.
This is a five-lippie product, and I say that with no reservation whatsoever. I can now imagine doing things with my MU that I couldn't do before out of fear of having it dripping onto my cheeks inside four hours. If you wear eye makeup at all and don't own Primer Potion, get it -- it'll be the most beautiful fourteen bucks you've ever spent. (Well, the effects will be -- the packaging kind of looks like Barbie's First Vibrator. But who cares? What's inside is well worth it!)
EDITED FOR ADDITIONAL COMMENTARY, 1/30/2014:
Almost ten years ago, this was my HG primer -- I loved it. When I got back into wearing e/s, I bought a fresh tube, and then I discovered that UDPP was no longer a godsend for me. I was thirtysomething when I wrote the review initially -- and now, at fortysomething, I find this is too chalky and drying for sensitive, aging eyelids.
I haven't tried the anti-aging formula of this yet, but I can say that the 'original recipe' is no longer something I can see myself using, because it does settle into the lines on my lids and makes my shadow look awful, even when it was used with an eye cream underneath.
Because it doesn't work as well for more mature users, I decided to drop this rating a lippie. I'm pretty sure it's still as awesome for the younger folks out there, but I can't recommend it for over-40 users.