Hydration packs have been, and will remain, an essential component of ultrarunning.
I have to admit, I never got comfortable to being strapped and trussed with bottle or bladder packs. Some are better than others but most chafe, rub, get hot or obstruct movement.
At right is one of the latest double bottle packs from GoLite; it's a minimalist design that gets the job done. I have a similar model from several years ago. It seems to bounce a lot and needs constant adjustment. I also have an Ultimate Direction double bottle pack, two Ultimate Direction single bottle packs, and a Nathan two-liter back pack.
Does any out there have a favorite model that works for you? If so drop me a few comments, I'll post some product reviews ASAP.