I used Norfolklines, Dover to Dunkerque, due to cost. Dunkerque is about 25 - 30 miles in the wrong direction... It was £180 for car + caravan and takes 2 hours.
Frequency of ferries is less so unless you are really early you won't get an earlier one. Likewise, if you miss one, it's a long wait for the next.
The service is designed for trucks, and the food reflects that - good food, cheap and plentiful (although one meal was a little cold).
The truckers have their own lounge with free coffee etc. The passenger lounges are smallish but plenty of room for the few passengers... Drinks were reasonably priced, and they would refill your coffee for half price.
Check in at Dover was OK. However at Dunkerque it was a nightmare. The signage is not good around and in the port. They have a "queue and hop on" service for trucks. So there is a huge truck park for those waiting, and there was a very long line of trucks (a mile or more maybe) all the way along the access road to the port.
Luckily when we joined the back of the queue we realised that we had missed out sailing (

) so I called Head Office who swiftly arranged for me to go on the next sailing, and told me to drive along the wrong side of the road past all the waiting trucks to get into the port. Otherwise I could have been there for days - you don't deliberately get yourself into difficult positions on the road when towing!
I would use them again if pushed. The long crossings are cost prohibitive for me with the 'van, despite the additional driving time and fuel ( - I get free fuel in the UK so tank up at Dover, and arrive back at the French port on vapours). P&O Dover were way too expensive in January when I made the booking. They later brought their price down to about £180 but the Norfolkline ticket was non cancellable. Door to door journey time was about the same using Dover or Portsmouth IIRC.
I've used Speedferries a number of times for work/pleasure but they won't take caravans. I believe they are getting another boat for the summer...