I'm a bit of a bag enthusiast, so I clicked through when I saw the query. Unless your commute ride is short, I'd recommend against a leather messenger bag. It's not going to breathe well. It's going to be heavy. If it isn't heavy, it won't last very long. Brooks bags are gorgeous. I wouldn't want to ride with one for more than a short way.

I'm a big fan of Saddleback bags and use them off the bike a lot. I also have a custom bag from "Don't Mourn, Organize" which is beyond gorgeous. When I commute, though, I use my Crumpler Complete Seed (if I need to bring clothes), my small Vaya (if I don't), or my GoRuck Echo. I also like BaileyWorks a lot for messengers.

Of course, feel free to ignore me and buy any one of the gorgeous bags recommended above. You'll have a great bag at the end.