I recently bought a used Spider Pro system from TP classifieds and finally got the opportunity to use it yesterday for the first time. I spend about five hours walking around a motorbike show with a 1Ds II and 24-105L attached. I found it much more comfortable than having the camera on a neck strap, bouncing around off your chest.
All in all, well impressed and I will definately be using it on holiday rather than my OpTech strap.
All in all, well impressed and I will definately be using it on holiday rather than my OpTech strap.