Used CPS on all four cruises, usually get it as part of the booking vice less onboard credit if the maths works out. Drop car off, I always take my bags over, it's not far. As someone said above the Porters are there for those that need them. We travel light, one case each and one small electrical items and charger case goes into the hole and then hand luggage only. Done hand keys over to being on the ship in 14 mins.
Likewise on disembarking, took ourselves off Britannia, had a disembarkation window of 07:30 - 07:45 and was north of Newbury on the A34 at 07:45. Really smooth.
Had one car clean and wash, that was excellent. Also as someone pointed out above we had a terminal change on the way home once, car moved for me.
Personally can't fault them. Like anything you will find cracks if you go looking. They may not be cheap but service is excellent.