Rode with Revel today from CH to EWR, the entire ride was free, and still plenty of credit left over. If you do give a tip, the tip is charged to your credit card, not your available Revel credit.
According to the driver they have somewhere between 200-220 cars and that this past weekend (Kinus) was busy nonstop and many people told him how they got it through the $400 in free codes
If you have an early morning flight, note that Revel is not currently available between 2am and 5am. At about 5:03 it allowed a ride to be requested.