I just got an Ortlieb messenger bag, size small. Although I have panniers, Carradice SQR bag, etc., sometimes I want to use a bike that doesn't have anything on it. Also this bag is 100% waterproof with a roll top closure. The suspension is very secure, holding the bag in position nicely. This bag may be small as messenger bags go, but it is still quite spacious, holding at least as much as a small backpack. The best thing is that it cost only $29.95 on clearance from sierratradingpost.com. Just thought I would give a brief report.
Regards,
Raymond