i'm really into hydration packs because it's a lot easier to take small sips as i go; not having to stop and mess with a water bottle makes me more likely to drink regularly. it's also a bonus that they take up less room as they deflate, and storing extra empty packs is easy.

the main downside is keeping it clean (the mouthpiece gets gross over time, and i've seen funk growing in the tubes), and it's more susceptible to freezing in the winter. also, since it's harder to clean, i won't use electrolyte tablets or other mix-ins with them.