We haven't done a transfer in years. It is either a taxi, a limo, or shuttle van for us.
I have to admit that although I advise people to get to the port a day early if possible, I don't do it all the time. For example, with the cruises we take out of Mobile on the Holiday we drive down on embarkation day. But for us it is less than 6 hours away by car. Even if something happened with our vehicle I could still rent a car and still get to the ship. Of course, we DO get insurance every time we cruise - even out of Mobile.