I always give 5 stars and a tip. Since Uber considers 4 a "fail" I feel pressured sometimes to give 5 stars for 3 or 4 star service. I wish Uber set the cutoff to 3.5 stars so that driver ratings are more accurate. I have only given one 1 star rating, and that was because the car smelled of cigarette smoke and was dirty and the driver was also on the phone.
4 stars = vote to fire the driver
If all the good drivers are fired, who will be left to drive me around