I use it with either one of my MBPs - depending on which one I decide to take with me. The weight is a drag (obviously) if I don't have a carry-on and so I have to put it in my Booq shoulder bag. When I have a carry-on, I'll just put it into one of the external zippered pockets for easy access during the flight. With the carry-on, the weight of the battery is a non-issue. The convenience of being able to significantly extend the battery life of my laptop is irreplaceable.
Also, I got a nice Waterfield pouch for my HyperMac battery. It prevents it from getting scratched-up and it has a convenient zippered pocket for the power cable. I got the large pouch for my 150 Wh model:
http://www.sfbags.com/products/gearpouch/gearpouch.htm