Well, in my humble opinion, based on trying to ride down the pier three years back...as the planks run down the pier rather than across, wheels get caught in the ruts unless you can ride very straight indeed, there's speed bumps (and apparently a 10 mph limit anyway, which is enforced even for cyclists). It was an awful experience on 40mm tyres, on 23mms it would have been terrifying. For the Randonnee (or just following the official round the Island route), it's off the route and three miles from the Wootton checkpoint. The twenty-five minutes you save in crossing time are wiped out by the bother of getting off the pier and back on track. Getting the car ferry's a much better option as far as I'm concerned- roll-on and off, a mile from the route, and it's cheaper to boot.
Might be an idea to go round again, I've not done the route clockwise. Thanks, I'll bear that in mind.