I've only been once, but I fondly remember it for having some of the best food I've eaten on holiday and all at very reasonable prices for what you get. Didn't have a single meal that was below great.
Snack time jam doughnut (the best doughnut I've ever eaten):
Cukiernia Pawłowicz
Chmielna 13, 00-021 Warszawa, Poland
Really lovely steak restaurant (the tastiest steak tartare I've had):
Butchery & Wine
Żurawia 22, 00-515 Warszawa, Poland
Gorgeous wine paired tasting menu (since got a Michelin star so probably crept up in price):
Atelier Amaro
plac Trzech Krzyży 10/14, 00-507 Warszawa, Poland