Well about mileage, I am getting between 16 and 18 (manually calculated). And drawbacks include poor interior and plastics quality, and squeaking noises in the cabin after 4-5k km, but as far as I know WagonR will have similar build quality.
Rest, seats are good (not excellent), and have done 1 motorway trip on it, feels solid at 120-140, drives well, AC is amazing, and mileage is good too.
I have heard that WagonR has slightly lower mileage due to being heavier and less aerodynamic, but they both have the same engine. I'll let others who have owned WagonR answer here. Mileage also depends on driving style.