We used Pop’s on our visit at Eastertime 2018. Their price was the best we’d gound, plus they had a location in Ermioni, which was convenient for picking up our rental car after coming from Athens and then the island of Hydra by ferry.
The staff at that office was slow, so picking up the car took a while. Not sure whether it was just that location, and other Pop’s offices might be faster with orereserved cars. They also didn’t have a map for getting out of town or the rest of The Peloponnese or Greece, just a local Ermioni map, so we had to find a bookstore that could sell us a map, and the owner had to go to his house several blocks away to get his “summer” supply of road maps, as they didn’t keep them on the shelves in winter and spring. So you may want to have your own navigation materials or systems, as Pop’s might not be much of a resource.
Turning in our car in Athens was quick and easy. Car was in good shape, too, so overall Pop’s worked for us, after an awkward pick up experience. If Sixt is $15 a day cheaper though, I’d probably use Sixt instead of Pop’s.