I booked a shuttle, and was given a 15 minute time window of arrival. I arrived a few minutes into this window, after speaking with the driver and confirming I was on my way down, and the shuttle was gone. I contacted customer service, and they refused to reimburse me, stating I was not there at the start of my time window (granted I wasn't, but they did not wait more than 5 minutes despite knowing I was coming).
There are better options that will actually get you to the airport.