When you come to Poland, you pretty much have to try a milk bar, and this is one of the more…read morepopular ones in Warsaw. Some of the older reviews cite a complicated experience with potential language issues, but that's absolutely not the case now. The wall menu has English translations, you order and pay on entering, and are given a beeper to take to your table and wait for you to be summoned to pick up your food. Basically, like any fast-casual dining experience anywhere in the modern world.
It's all very cheap, and it's a good way to try a sampling of Polish dishes. However, while the food is tasty and likely authentic, ingredient quality is understandably so-so (for instance, the popular veal schnitzel is made of a pressed/formed meat patty, as opposed to an actual slice of veal). For me, it's the kind of thing you do once, but then look for better quality versions of the dishes in one of the many good restaurants around town.