I've been using the harness for some time now,
and find it superb!
The sliding system enables you when you go for a wave to actually extend your turn,
since you have a longer depower curve with the bar.
The side you use your harness depends on your favorite wave approach,
if you hit a wave both directions, then the sliding system only gives you a longer depower turn one way (logic)
When riding normal, the hook stays more or less in the middle
when you try to ride more upwind, the slider actually helps in this
since its now also more on your side instead of the middle.
Give it a shot, you for sure have to get used to it,
specially since your normally stopped in a wave cause of the center hook