I believe it is a service that contracts with independent local delivery services/drivers. At my home airport (SAN), I have had uniformly bad experience - some of it mentioned above - like a long delay for assigning to a driver, inaccurate estimate of delivery time, the website link supposedly showing the driver location was wrong, etc. I received bag 30+ hours after my arrival at SAN from JFK and 18 hours after the bag arrived at SAN. There was a phone number provided on the website when it told me the bag had been assigned for delivery, but it simply went to a voice mailbox and I never got a response.

In other towns, I have had excellent, timely service. Again, I'm thinking it is highly dependent on the quality of the local contractor.