If you have a triathlon bike or have added aero handlebars to a road bike (so that you can ride in a lower position resting your elbows on the bars), then it is really helpful to have hydration readily accessible. Amazingly, I am not the first to realize this, and there are a number of bottles on the market for this purpose. Some can be installed right between the two handlebars as an extra bottle. Others sit in a normal bike cage and use a straw that can be attached in a convenient position to sip from.
On my bike, I do not have room between the bars, so I have used a system that uses the straw. It is super duper awesome and I am sure that I drink far more and drink more consistently throughout my ride.
Any system should be designed so that they are really easy to fill from a normal bike bottle. They have an open area where you shove the top of a bike bottle in and squeeze your liquid out. This allows for easy filling on a long training ride or during a race. (Imagine going through an aid station, grabbing a bottle, using it to fill your special hydration system, and discarding the bottle).
This is what I have and love. I suspect that all models should work pretty much in the same way. Also, to attach the straw to the handlebars for the model that I have, whatever it came with didn’t work very well. I purchased some small velcro straps on Amazon that have proved really handy to affix the straw easily in the place that works best for me.
Another note on my model, I haven’t found a great way to clean it. I think it is time for a new one, but I have used it for 2 or 3 years.