Dog Backpacks

I'm planning on getting a backpack for my adult lab in the next few weeks and one for the puppy WHEN SHE IS OLD/DEVELOPED enough. No way would I put one on a 5 month old puppy. Just though I'd clarify that.

Anyway...which brands have you all had success with? I have seen reviews about how some sit too far back on the dog and possible back problems...but of course these are the ones that seem the most readily available right?

So...I'm open to suggestions. Both dogs are labs. The current adult is about 50lbs and the puppy is projected to be between 55-60 when she is fully grown.