I've got the us20 and two us 10's, I've only ever used the 20 and one 10 and that'll last me a week. That is just clothes, shoes and wash stuff though as I would put my tent in either a waterproof roll bag or duffel. The trouble with them is that to get anything out you end up taking out everything, not so bad when you stop for the night but a bit fiddly if you need to get something en route. I do like them though and have used them on sportsbikes, cruisers and off roaders. I also agree with Pleiades in that they work much better when full.

I've got overboard stuff as well and can vouch for their quality. A roll bag like Pepsi's would be far easier to retrieve stuff from and probably a lot cheaper too.