I'm just finishing up a 7,000+ mile trip on by BMW K1200LTC. Of course I used Tom Bihn stuff for carrying what I need on this 5 week trip.
Here is my Aeronaut and Brain Cell in the back compartment of the bike. The Brain Cell accommodated my 13" MacBook Air and my iPad just fine.
However, I found that after a bit of riding this placement of the bag unbalanced the bike, so I bungeed the Aeronaut to the back seat and left the Brain Case in the compartment.
Now for the rain! The Aeronaut is not waterproof, but I went through one major thunderstorm. I placed the bag with the zippers facing down and much to my surprise the interior only got a little damp. At the hotel it dried out in about an hour.
One major plus of the bag, as motorcycle bag, is that the bungee cords can pass through the grab handles and make for a very secure attachment.
You can even use the bottle holder as a convenient pouch to store the bungee cords.
If Tom could come up with a rain cover for the bag I bet he could sell it to the motorcycle crowd!