I have the Kriega OS Adventure system mounted on DirtRacks pannier frames:
The Kriega bags hold their shape, even when empty. I have seen the Givi system which is similar to Kriega, however, it relies on a big plastic panel which is built into each bag to connect to the pannier frame, making it quite bulky, in my opinion. The only advantage is that it locks to the prame.
It may have been a costly system but you get what you pay for. I knew someone who went the cheap route with the Lomo panniers and he regretted it as soon as he fitted them, loaded, to the bike: it was obvious that they weren't up to the job as he was using Rok Straps to hold it all together. I love Lomo for their dry bags, by the way.