If they are built to the same standards as the panniers then go for it.
I recently replaced a BikeHut pannier with a pair of Creek2Peak panniers (nothing wrong with the bikehut one, just too small).
The first time out with them I had forgotten to engage the clips, and the pack fell off when I hit a pothole, skidding down the road as it went.
I turned around, picked it up, fixed it back on and carried on my way. Once back at the digs I checked the bag out to find that apart from it being filthy there was no damage done.
It also keeps the rain out in all but the monsoon type downpours we had the other week (when I doubt anything less than a totally sealed bag would have kept things dry!)