Ziplocs bags are cheap, but I wouldn't rely on them if you really need to keep something dry. My experience with them has been inconsistent, about the only time I would count on them to keep things dry is if they bag itself never got wet. I think the abrasion of being in a pack wears tiny holes in them after a while.

I really like the Pacific Outdoor brand of compression drybags, which are really popular for canyoneering. They're much lighter than regular vinyl bags, probably about the same weight as most stuffsacks. But they're plenty durable, and the air valve (similar to a thermarest) allows you to control the amount of air inside. so for normal use, you could almost use like a vacuum bag to keep your stuff compressed and take up less space, or if you're doing a water crossing, you can inflate it and use it for buoyancy. A slightly modified version was used in the Arctic 1000, the details can be seen here

http://www.ryanjordan.com/2006_arctic/2006/05/backpacks_for_a.html