Whatever you do, DON'T buy the PNT add-on laptop bag. The problem is that, when you don't have your kitbag on it for commuting, the laptop bag doesn't touch the ground, so if you stand your bag straight up it will topple over backwards.
They REALLY should make some extension clips so you can let it touch the ground when standing straight up. Because of this, I don't even use mine anymore, I just carry a small light laptop padded zipper case with a handle.
Neither of their solutions seem to be very good for a normal regional guy who flies 4 and 5 day trips.