Enjoy the ride and hydrate
Designed with riding in rugged terrain in mind, the Shimano Unzen hydration pack hugs your body and allows you to move more freely, thanks mostly to how the straps are placed.
The straps form an X across the chest and are positioned right in the center of the pectoral muscles, which doesn't restrict movement like conventional straps do.
There are no loose straps that could mess the aerodynamics. They even have breathable back panels to keep your back cool while you're riding and enough pockets for your keys, tools, and other stuff.
The Shimano Unzen is a good hydration pack despite reports that the outer shell is delicate like in the following review:
"First, the outside fabric (the turtle shell looking motif) is very delicate and I've already torn two small rips into it. The first hole ripped during the first ride. The second one has developed over time. I've only taken it out for about six rides since i got it and I've already had to patch it with tube patches. It does stretch as Shimano claims but it does feel very delicate. Secondly, it is hard to get a full bladder secured into the compartment because of the way it zips on the side. It is not impossible, but if you're used to slipping the bladder in and out with no extra effort, this will be a nuisance."
If you're looking for a better hydration, there are plenty out there. The time for Shimano Unzen is done. Most stores don't even sell them anymore.