taxi drivers on rank right outside arrivals in Warsaw are metered - should cost around 30 Zloty to get to Sheraton - depending on time of day night (remember there are 6 zloty to US $ so this is cheap) the sheraton has cars to take you back to airport at a flat rate of 40 zloty.

i only had one taxi driver try to rip me off with a rigged meter (going from old town back to sheraton) he tried to charge me 120- zloty, i simply went to get the hotel doorman, and by time i got back, said taxi and zoomed off.

sheraton is fine, but can get a bit busy. make sure your room is not above the bar (even four floos above it is noisy)