Driving on and off the ferry is an 'absolute doddle'.
On most crossings you'll almost forget that you're onboard a ship - it really is that smooth. However, the worst crossing I had was on a SeaCat form Ostend to Dover. (Southwesterly winds, force 8, gusting to 9 or 10). We had to wait several hours before the captain was prepared to sail. Then the crossing took far longer than usual because it was necessary to go slow in the heavy seas. People were being thrown out of their seats, onto the floor and against the walls. There were two "is there a doctor on board?" calls during the trip. (I was on the lower deck. I understand that one person died of a heat attack on the upper deck, where someone also suffered fractures through being thrown cross the lounge by the force of the sea).
Happy sailing ;-)
Chris