That was my first piece of bike luggage...I used it for about a week before I really started to hate it (the one with the unfolding panniers). The way the bag interfaces with the rack is very smart and quick, but that's about where my praises stop. It's not so much that it's a terrible bag more than if you need to carry it off the bike when full, it's basically a nightmare. In my experience the bag, while on the rack was nifty. The trunk bag had lots of space for carrying basically anything a commuter would need plus a bit extra for a small bundle of groceries, change of clothes etc in the panniers if needed. But if you need to carry the loaded bag off of the bike for more than a few minutes, forget about it. The shoulder strap it comes with is minimal and very uncomfortable (has one of those soft rubber/plastic grippy things that are supposed to keep it on your shoulder). Also then walking around with the panniers down, they bounce around and hit you because they are basically free to flop back and forth when not confined to the rear rack. If you have no need to carry your things off the bike then I guess the bag is a good idea.
For my personal setup, in case you were curious, after having a few years of bike commuting under my belt I've found the perfect combination of rack luggage that works for me (of course it may not work for everyone). My daily tag-a-long is the Ortlieb Vario Backpack/Pannier. I'm a student and need to carry books, laptop and the like to school with me on a daily basis. The Ortlieb Vario allows me to carry it on my bike for the ride and then once at school convert to what is actually the most comfortable day pack I've ever had (thanks to the rigid back panel). Then I have two other luggage items that come with depending on what I'm doing that day. I use the Ortlieb Trunk Bag of I need to bring some extra clothes or a lunch. And if I need even more space for shopping etc., I use one of those fold-up shopping panniers. As you can imagine, when I'm carrying all three items it probably looks cumbersome, but the heaviest part of the load is always in my backpack (where it's easiest to carry) and then the trunk bag is slung over my shoulder and the shopping bag in one hand or the other. Even with all of this, I still prefer this setup to the Topeak trunk bag because it's separable and much easier to carry.
Some links to the items I'm talking about
:
http://www.bikebagshop.com/ortlieb-v...er-p-1720.html
http://www.bikebagshop.com/ortlieb-t...ag-p-1746.html
http://www.rei.com/product/825299/no...e-bike-pannier
For the last one (the shopping bag) I actually swapped out the rixen+kaul hooks for the ortlieb quick release type (in case you couldn't tell, I like Ortlieb's designs...a lot).