In terms of cost versus a taxi, it comes out about the same, the one disadvantage being that taxis come to you - you don't have to search around for one. Also, you can't sign out from the car outside city limits/their service zone; you're still on the meter until you bring it back. That said, as an alternative to car rental, Traficar is quite a wonder. You can rent a vehicle for 24 hours for only 89zł and none of the common shenanigans of car rental; plus you don't have to pay for gas. You can also drive and drop it off in another Polish city like Warsaw, Wrocław or Katowice where Traficar operates. As such, this is a great option for day and weekend trips, as well as big shopping trips, or simply picking up friends at the airport.
Two important things to note are that the app and website are only in Polish, and you need to have a valid driver's license that is recognised in PL. As with regular car rental, citizens from countries that didn't ratify the Vienna Convention on Road Traffic cannot drive on their home licenses; unfortunately this includes Americans, Australians and the Chinese, among others, for whom an International Driver's License is required.