I use them all the time, no drawbacks I've ever noticed. Most of what you read about them affecting the handling is bogus unless you're carrying bowling balls. I have two from Rivendell, a Candy Bar Bag and a Hobo Bag, wildly overpriced but I got them as gifts. CB is tubular, maybe 14 inches long and four in diameter. Hobo Bag looks like this:"
http://images.google.com/imgres?imgu...G%26ie%3DUTF-8. They don't carry it anymore, but they have a similar one.