In reply to Nomisr:
We have a couple of collies and bought the Palisades Packs from Ruffwear for them - it's essentially the webmaster harness with a removable set of saddlebags.
For our larger female collie, it's a great fit and she seems to really enjoy getting 'geared up' because she knows it means we're going out for a long day, or even better, multiple days.
The smallest Palisades is too big for our little collie - she's very small for the breed, but I couldn't get it cinched down to be a snug fit on her and it seemed to rub her neck. I've abandoned it on her for the moment, and I'm going to have to go back and get a smaller harness for her.
From our experience, I'd say that getting the right size is pretty crucial - but they definitely work for making life simple on scrambly bits, and at fences and stiles, where not having a wet muddy dog smeared down the front of your jacket is a real 'win'.