I use a handlebar bag on my Surly LHT. I find it to be perfect to carry lunch or a snack, my rain jacket and helmet cover, wallet, etc. The one I have is pretty big, but one advantage to that is that in colder weather I can ride with my hands on the tops of the bars and they're behind the bag and shielded from the wind.

I do find that I can't lean the bike against anything, as the weight of the bag causes the front wheel to flop to the side and the bike tends to fall over. I always have to lay it down - no kickstand, and I'm not sure the same thing wouldn't happen if I had one.

I don't worry about the aerodynamics, as I'm 6 - 2 and 260 LBS with 24" wide shoulders - I figure the bag has moot affect on my aerodynamic profile! (My triathlete neighbor says he want to ride with me....said something about drafting a bus.)