I used to ride with a backpack, then moved to saddle bag/stuffed jersey pockets but after treating myself with a decent hydration pack I don’t think I’ll go back to “gigantic jersey pockets” again.
I drink a lot, one bottle is almost never enough for me. I don’t like thinking about the strategy when/where to find a place to buy some fluid (if it’s actually possible). I don’t like microscopic pumps, so the big’ish one lives in a pack. Plus tools, gels, wallet, etc.
The pack I’ve got is this: and it’s extremely comfortable!