Here's my reply to a recently asked similar question:
"It's nice having somewhere to stash things, but I think I'd rather keep all the weight off the handlebars & steering, although I admit that I've ridden many miles with a handlebar-bag.
What eventually turned me off the practise was that I was descending a fairly smooth section, had got up to about 35 mph and suddenly hit some bumps; the bike shook, rattled and bounced so violently that the bag flew off its bracket, fell down and got jammed on the wheel but behind the fork, in front of the down-tube. To this day I still don't understand why the bag didn't fall into the wheel; I was able to stop without crashing, but if the bag had fallen into the wheel and jammed it, the story would have had a far worse ending, so be warned, make sure the bag is very firmly attached!"
Take heed!
- Wil