Can't comment on carrying the bag off the bike as I have no need to do that. The MTX system works fine, and I do take the bag off for pleasure rides. I have the model that the top expands, as well as the fold down panniers (DXP? maybe). Its a bit more money, but I really like it. Most of the time the main compartment, sometimes expanded, will carry what I need. The panniers are great if you pick up something unexpected when shopping. Also good if you need to wear a jacket on the morning ride to work, but not on the ride home.
http://www.modernbike.com/itemgroup.asp?IGPK=2126176145
You may be able to find a better deal. I just wanted to show the model.