I just fitted the sw-motech engage/enduro bag on my GS this morning.
Great bag so far, easy to fit, though slightly fiddly as you have to bolt the mounting bracket onto the bottom of the bag yourself (though this does mean you can place it so it sits exactly where on the tank you want).
I was a bit sceptical about the mounting system before I saw it, but now I have it, it is very impressive. Very easy to attach the bag, very easy to remove it, but fits very solidly and snugly.
Can comment on waterproof as it didnt rain today, but it seems that it would only be shower proof. It does however come with an elasticated waterproof showercap style cover.
Quality of the bag is great. Inside it has two sections with a divider between them (front/rear).
I got it to carry an autocom, radio, ipod, phone, wallet and smokes. Its actually smaller than I thought, so that is pretty much all it will carry.
Rich