I have a backpack for my two.
I bought it at a dog show so I got to try it on them before I bought it, although it does have adjustable straps. Neither mind it, although Emily doesn't like it that much when there is a water bottle in the pocket that runs along her back.

I believe the general rule is that you can work a dog up to carrying 10%-15% of their body weight.

It doesn't hinder my dogs at all, infact it doesn't even slow Clipse down, nor does it shift when Clipse is running (and if you've seen pics of him running you'll know how amazing that is).

Anyways here are a couple of pics of my dogs in the backpack.