I've got a Kriega UScombo40, two US10's and an US20 bought as a package deal, and I love it. The bags are completely waterproof and they're very flexible. The only downside, if any, is that it's the tiniest bit difficult to get into the main compartment, when you have the bags on the bike, because of the rolled top.
I've been using one of the US10's and the US20 separately on shorter trips and have been surprised by how much stuff they hold. I've no trouble fitting either to my FZ1S, but using the UScombo40 you have to take care that the bags don't rub on the passenger handles.