I second the Topeak MTX series. I've been using the big one (approx $100) since June and love it. I routinely carry a spare tube in its box, patch kit, tire levers, Power Bar, multi-tool, wallet, watch, cellphone, pair of socks, underpants, pair of jeans, undershirt, regular shirt, and a few other small things. I've never had to use both panniers and expand the main compartment all at once. I have fit a 13x9" brownie tray in one pannier and gotten it closed, too. The material is very sturdy and has a reflective stripe on each side. I love that I don't have to mess with velcro to attach it to the rack.
The only drawback I've found is that I can't use it with a grocery pannier (or maybe I just haven't tried hard enough)
It is pricey, but I don't see why I can't use it for years to come. I just need to figure out what to do with the bungee cord setup on top.