I've got the Topeak MTX rack as well, along with their mid-size bag that locks into the MTX rail. I like it, although I almost never fold down the panniers out of the bag. I've never had a problem with the bag coming unlocked while biking, although it can make a soft rattle at times if you've got almost nothing in the bag and it's very light.

I actually also have a set of Arkell Shopper panniers that I use about 2/3 of the time instead of the lock-in MTX bag. Really, really like those panniers...love the easy-on easy-off camlock handle system, etc.