I only use my hydration pack if I feel like water bottles aren't going to work. It is somewhat uncomfortable on my shoulders. The nice thing is that I can get almost 100 miles out of it if it's not too hot. I think hydration packs might lead some people to drink too much water, I know one person that says they lead to a higher DNF rate. I'm not sure about that, the conditions where I wear a hydration pack lead to a higher DNF rate no matter what I'm wearing.

The most frustrating time I had with a hydration pack was when I was out of water but there was still ice in the pack. So I had to wait for some of the ice to melt to get a drink. It was really hot that day, i was parched.