The TorHans company is bringing some thoughtful engineering into the world of aerobar-mounted hydration. I took an in-depth review at their Aero 20 product, which is both easy to use, and tests very well in the wind tunnel. See the article for a link to their wind tunnel data.
The number of legitimate hydration options for triathletes has exploded in the last 2 years or so, so much that it's become ripe for a real survey-type article. This is our 2014 BTA Hydration Shootout.