I used ExecuCar for two airport trips (to/from) in December. I had reserved a sedan, but I was sent an SUV for my trip to the airport. A nice surprise! It was a Chevrolet, very clean, courteous and well spoken driver, and he was on time. Also, he had an iPad mini that had all my flight information, which was very convenient.
Coming back from the airport, however, was not a pleasant experience. My driver had been given the wrong information and went to a hotel instead of the airport. I waited for 30 minutes after getting my luggage for him to get to the airport. The only reason I waited instead of taking a cab was because I had already paid for the car. When my driver arrived, he was very kind and apologized profusely. The only complaint I have is he was quite chatty, which did not fare well after a 9-hour trip from San Francisco.
Based on the two trips I had, I would recommend ExecuCar for infrequent, casual use. However, if you're on a tight schedule and can't afford having your car sent to the wrong location, I would recommend choosing a different car service.