I used the nashbar waterproof front panniers for a year. I'll echo the last fellow and say they seem durable enough. They have a sort of double closure - you roll it like a dry bag, then pull a waterproof hood over that. The space under the hood is nice for sticking extra water bottles, as it keeps them from sweating on your dry stuff.

I got rid of mine because they just don't mix with a Tubus Tara front rack, and I'm hardly going to replace a $100 front rack to accommodate $20 panniers. Every time I hit a serious bump they'd fly off, which made for some pretty tense riding. My mountain bike has a Topeak DX rear rack, and the Nashbar attachment system works fine with that. Just got to make sure the whatever rack you pick has a stable spot at the bottom to hold the hook.