I bought the Mystic Code 01 6 months ago, the reason for me buying this one was back pain when I spent many hours on the water day after day. Back pain completely gone now! :) And...
No slipping also. Overall I really like this harness. I got the kite version, works perfect for windsurfing also, there's a windsurfing version also i you prefer that.
A great feature is the adjustable back support, you can adjust it quickly and easily on the water or wherever, just turn the dail until the support you want is strong enough. Only thing is maintenance, you gotta rinse it with fresh water the dial after each session but no prob to me, life long warranty on the system also.
The harness also has a kite line cutter thing which could come in handy if you surf in an area with a lot of kiters.
See the pic, top left is the cutter thingy built into the harness for you to use at you leisure.