In a word, awful.
The Pop Burger is two small separate mini burgers boxed up side-by-side (not normal size patties stacked on the same bun). I guess it was a brioche? Extra bready bun, in any case. They don't skimp on the bun volume (sarcasm). The meat patties were small and relatively flavorless, about the same as the beef you get at a fast food joint (but with gourmet prices... $8.25 for the Pop Burger(s). They also had some shredded lettuce and 1000 Island/Secret Sauce. I'm not sure why they choose this format (tandem burgers) -- maybe it's supposed to be cute, or easy to handle. The Pop Burger was not good, in any case. Edible is the best thing I can say. Big bun and small, flavorless meat.
The french fries were good, I'm guessing they were battered and twice-fried? I'm not sure how they were prepared, but not unlike the style of fries at Popeyes (minus the Cajun spices) for a ballpark comparison. $4
Possibly the worst milk shake I've ever had. Certainly the worst one that I can remember. Lets see, foamy (lots of air) and it tasted like it was made from a mix. Weak, artificial chocolate flavor. Worse than what you get at a franchise. Much worse. $5.75
Now maybe I ordered the wrong things, but I assumed that the namesake hamburger for 8 dollars would be a safe bet. And the decor boasts "frosty shakes" or something along those lines (big cutout steel sign). But this food was lousy. And my bill was over 20 dollars... outrageous.
I don't mind dropping a bundle on a hamburger and ice cream, I do that at the Shake Shack every couple months. I'm willing to pay more (a lot more) than what you pay at a franchise (Wendys, etc) if the food is really good. Unfortunately, this wasn't good. I honestly feel ripped off. It's more expensive than Shake Shack, and not nearly as good, not even close. It's not even as good as Wendys or Burger King. Even McDonalds is better.
I should mention the music was loud, to the point that it almost drove me out of the restaurant. Pounding. I noticed a lounge in the back when I was washing up, and I just checked out the Pop Burger website where they tout their lounge. So maybe that's their real business. But the food up front was dreadful. There's no reason to blast music in the restaurant section, but this place seems to be all about style over substance.
I only went here because Chelsea Market was closed (I like the Lobster Place and the Thai kitchen, and I was in the neighborhood). Never again.