I've got this bag. It's pretty good, with a couple of minor reservations.
The good: the interior is pretty big, and can easily hold enough kit for a week away (more in summer). The base is semi-regid, so it's a bit tough, and it stays in place when straddling the rear rack or seat. It's also completely waterproof - I've done multiple motorway hours in heavy rain and nothing inside was damp at all when I got to my destination. There are quite a few pockets too, for odds and sods.
The bad: it's got an elasticated loop on the bottom that is supposed to help secure it to a BMW rack, but it does not fit my TC GS at all. This is probably why the bag does not feel particularly secure when using the supplied straps - I had four in use, and there was sufficient movement of the laden bag that I didn't want to trust them to secure everything on their own. So I suppliment them with a couple of bungees, and then it feels very secure. The bag is also not exactly 'stealth friendly' - bright silver does make it easier to locate your bike in the car park though. Oh, and new they're pretty expensive, but then the bag works very well and has lots of nice details that make using it a pleasure. They do come up on ebay sometimes, but I wouldn't hold your breath (and they tend to go for good money there, too).
I tend the use this bag if I'm riding somewhere direct, that I will then use as a base of operations (so I don't have to worry about an insecure bag left on my bike; paranoid, I know). If I know I'm going to be stopping lots and leaving the bike, then I use the panniers.
Hope this helps.