I've used the SW Motech tank bags for my last three bikes - Kawasaki 1400 GTR, Kawasaki Versys 1000 and now the K1300 GT. Each time that I've changed the bike, I've needed to buy a new tank bag and tank fixing ring, however the convenience makes it worthwhile to me. The old equipment has sold in no time and remained in "as new" condition throughout my ownership.
I've tried alternatives, however nothing comes close to the SW product for my use. Once fitted, I only take it off the bike to refuel, although I don't really need it all of the time.
Over some five years of using these bags, I've never yet found any water penetrating the fabric, despite touring in all weathers.
Highly recommended in all!