Liesl,
Wow does that bike look great!
Sean,
I have the front HAR/HAB. I've had water enter the front bags in heavy downpours (when commuting). I put my clothes in plastic bags... problem solved. As it's near the front hub, there's a lot of water exposure from fenders, wheel, brakes.
Liz,
It could be easy on/off. It depends on how you set up your strap(s), which might depend on the type of rear rack. (I'm speaking as a user of the front version, so I don't have direct experience with the back.)
Overall: recommended-- simple, attractive, well designed, durable. (Seems like it would be easy to repair, but I haven't had to do that yet!) But if you're looking for waterproof, get Ortliebs or other model.