My experience with SuperShuttle wasn't too bad.
They picked me up at the end of my time window. There was a total of 3 of us in the van (including myself) and I was the 2nd to be picked up at 5:45AM. I originally told them my true flight departure time of 8:45AM but after seeing the latest time window they could pick me up I was worried about making my flight so I lied and said my flight left at 7:45AM so they would pick me up much earlier. I would rather arrive at the airport too early than too late. I would advise using a similar strategy when telling them your departure time.
The driver wasn't that bad but it was so early in the morning that there weren't too many people on the freeway so who knows how he would do with a lot of traffic. The one thing the driver did that I didn't feel comfortable with was multitasking and using the computer while driving on the freeway to the airport.