Zeebrugge was, I believe, very popular with all the turmoil in Calais this summer, with apparently a jump in numbers. I think Zeebrugge is handy. But your right Steven efficiency is everything. I'm not a fan of the Vissentini's but I do have to agree, if Stena Mersey and Stena Lagan were put on the Hull to Zeebrugge they would probably cope fairly well, and would be cheaper to operate.
One design I was always quite impressed by was the Isle of Inishfree (Pride of Cherbourg, Stena Challenger). She seemed to combine comfort and efficiency quite well.
I'm wondering what form the next generation of vessels will take, not just for P&O, but also Stena. I have to give P&O's Mitsubishi's credit as-well in terms of effective use of space.