I've heard people don't like 'em, but I've got a set of Chase Harper Stealth bags for my bike. They are nicely shaped, albeit a bit small, and they hold their shape when empty giving the appearance of a good hard bag. They don't come with rain covers, so you've gotta "garbage bag" everything inside. I rode for two hours in the pouring rain, the bottom of the bags were SOAKED- but there were no signs of moisture at all inside the bags. Chase Harper says the Stealth bags are constructed of a proprietary material and call them "water resistant"- to me, they feel like thick neoprene (wetsuit material) on the inside and cordura on the outside. They were sort of pricey compared to other bags, but have nice lines to them. I packed mine completely full and couldn't even tell they were there. Mounting is simple and features a quick release set up. Take a look at them, see what you think. They were enough for me to take a week's worth of gear in tow.
Although, I don't usually ride with any sort of bags at all, so YMMV.