Just done Birkenhead - Belfast with them and was also impressed. We booked a cabin on the (overnight) journey out but not for the (day) return. However at check in coming home, they offered us a cabin for £20 as they had loads spare, which we took, because 8 hours is a long time to hang about and I fancied a nap. They upgraded us to a premium cabin - which as far as I could tell was identical to the normal one we had on the way out, but the carpet outside was different and inside it had one of those capsule coffee machines (which made coffee that smelled off so I bought one instead) and some biscuits (which were very nice).
Having a cabin on the way back was ace - well worth £20 to have somewhere to nap / watch the Olympics in peace. Even the food was better value than I was expecting. Not cheap, but not a complete rip-off either.