I owned this bag for a few years, but didn't use it a whole lot as I wound up not traveling much during that period. I normally use a shoulder or small sling around town.
As usual with Think Tank gear, the build quality is excellent. The shoulder straps are nicely padded and very comfortable. The pad for the lower back is hinged and you can thread a Think Tank belt through it to carry more gear around your waist. I didn't use this in practice, but I did test it out and it looked like it would work well. The lower back padding is soft and breathable, but the mesh did get full of sand once and it took a while to get it all out. The bag also has plenty of zippered compartments to organize your gear.
Compared to most backpacks, one thing I liked was the fact that each lens or camera a body had it's own secure pouch with drawstring. Access was a little more time consuming, but this design virtually eliminates any possibility of accidentally dumping all of your gear when opening the bag. I imagine this would also hinder a thief trying to quickly unzip and grab an item.
When not loaded with camera gear, the bag's second zipper compresses it to a very thin profile. You could still carry a laptop or stuff in the outer compartments in this configuration, but in practice I only compressed it for storage.
While the "every item it's is own pouch" system has it's advantages, I did discover one flaw which lead me to sell the bag. I got caught in an unexpected rain while shooting an air show last fall. It was still raining when it was time to pack up and go. Problem: Stowing my camera and lens in the Shape Shifter requires dismounting the lens, exposing the internals to the elements. Thankfully wasn't a downpour but a traditional camera bag would have let me drop the whole camera+lens combo in more quickly without opening it up. This shoot was at the beach which reminded me how useful "rear access" backpacks can be to keep your clothes clean when the bag must be set down in dirt, mud, sand etc.
I hadn't taken the rain cover with me but the bag still did a good job keeping out the rain without it. I had to walk about 3/4 mi back to my car in a pretty steady rain and the fabric got a little damp around the zippers, but there was no major water intrusion.