I have the Topeak MTX which I really like... BUT like mentioned before it can be used on a Topeak rack only. I just bought a Trek 520 which came with a rack. I then got the Ortlieb Backroller Classic Panniers - Pair from REI on sale. This is my daily commute set up. I carry all my cloths to work, shower in a nearby gym. I wanted the easy access of the MTX trunk bag so ended up drill out the Topeak MTX slide rail so I could ty-rap the trunk bag in place.
After looking at a friends seat post QR mounted Topeak rack I am seriously thinking about buying one to take a part and then mount the rail onto the Trek 520 reak rack.