Has anyone used those back-pack hydration systems? I've used a hip-belt/bottle thing for my long runs for years and I'm curious as to how effective the packs are. The belt isn't very comfortable until I've got the bottle half finished. I saw a CamelBak Hydrobak cheap (a small unit, 50 oz. I think) and was wondering if they're stable and comfortable. As I go through a 20 oz bottle in about 8-10 miles, I thought this could be just the ticket for my longest runs.