I used to commute to work on a motorcycle and used a courier bag. It is quick on/off, although off the bike, it isn't as comfortable as a backpack if you have a heavy load. For that, I use a standard backpack. My current one is a Timbuktu backpack that I bought used on Ebay for $25shipped. It is built as well as any other bag I have seen. Because is is a 'fashion' brand, I think it is low profile for the most part, even with the camo high accents on it.
I use either a Maxpedition or a non-gun specific pack (Aerostich, Timbuktu, Vans, Dickies) courier bag. Depending on the gun, you don't need a gun specific pack with velcro. In a holster, a full size handgun typically fits in the outside pocked on a backpack or in one of the organization pockets inside a courier bag. It also makes it easier to not have to unholster it and put it into another holster.