When I've used them to and from the airport, I tipped $10 each way. That's with 5 or 6 pieces of luggage. We always got great service, so I thought they deserved it.
I think we did 12 or 13 each way last time. You pay the whole round trip fee when they drop you off at your hotel, so for the trip back you just need your tip! I put it in an envelope and leave it in the room safe, so I don't forget and spend it!