Jon's review is right on the money. I've been using the original Turnstyle 20 for over two years and it is my favorite bag for in the field. My routine is to carry a camera body or two outside the bag, and carry three lenses plus accessories in the bag. That lets me leave my full pack of gear in the car and just take what I need into the field.
The Turnstyle 20 is my bag of choice. It holds the Nikon 16-35, 24-70, and 70-200 f/2.8 or equivalents. The bag is deep enough to hold these lenses in any slot separated by the dividers. I might substitute a 105 macro, a 300 f/4, a camera body, or several smaller lenses in any of the slots.
The large front pocket holds plenty of batteries, cards, business cards, pens, etc. I've used the pocket nearest my body for my iPhone and wallet, or for a poncho if I'm in wet conditions.
The update to the Turnstyle 20 V2.0 has a number of small changes and updates to an already great bag. The fabric on the back side has been updated, the felt lining of the tablet pocket is new, and the dividers are improved. This is another great bag from the folks at ThinkTank.