I prefer Bobak's pierogi to Kasia's. Bobak's has a sauerkraut and mushroom dumpling that is fantastic. What a great blend.
Given that my wife is half-Polish, we don't eat it out often enough. Some Polish acquaintances recommended Tatra Inn on South Pulaski, which we love. As Han Solo said "she may not look like much, but she's got it where it counts."