Messenger bags suck. They're not comfortable for heavy loads or extensive riding IMO.
The best messenger bag is Ortlieb (backpack), which I own and use <10 miles no problems. The Ortlieb messenger bag isn't comfortable after 20 miles for me, depending on the weight in the bag.
I also own two different TimBuk2s which are not comfortable at all because of the one strap design on one shoulder (terrible for your body too). For me these bags are nearly worthless and I'll probably dump them on CL.
I also own a
Seal Line Urban Backpack which isn't as nice as the Ortlieb because the waist strap is thin and cuts into the flesh more.
I recommend Ortlieb panniers: far, far better for any long commute with heavy loads. You can store the ulock on the rack w/ the panniers. Your neck, shoulders, and back will thank you significantly.