I have one and use it for almost every race over 50 miles, at least if it’s hot and water isn’t plentiful on the course. It’s a Nathan HPL 020, which is marketed towards runners (also why I bought it) but it does double duty on the bike just fine. I really like the shoulder strap pockets up front, which can hold my phone or some snacks. Capacity is 2L which is roughly the size of 3 of my water bottles. There have been 50 mile segments at DK that I’ve essentially drank all 2L of water plus the 2 water bottles of electrolyte mix I carry on the bike, so it’s definitely needed.

My only complaints are that Hydration packs are a bit of a pain to clean and dry for storage, and typically you don’t want to put anything other than water in them. Some people are annoyed by having the weight on their back, especially when it’s hot...I don’t find this to be much of any issue for me personally.