I don't drink it while I'm working out. But last weekend, for instance, I was helping out while my shihan was teaching a kid's class at 6 pm. At 7 pm I had to teach a 2.5 hour seminar. So I was looking at 3.5 hours of training and some degree of exertion (although the kid's class is more like a gentle warm up for obvious reasons). So being that I am prone to dehydration and cramping (I really sweat easily) I made sure I had a lot of water in me before class. I had my half banana as well. At a quick break after the kids but before the seminar I had a small bit of chocolate milk since it also helps keep the blood sugar, etc. more stable. I always have a toothbrush/toothpaste kit in my bag so I also brushed my teeth as it's the milk/chocolate mouth that I find kinda gross. I felt great through the seminar taking a few water breaks along the way. A jug of chocolate milk afterwards and I felt good to go.
One point of all of this is that there is more than just "getting water" into your system. There's issues of restoring glycogen to the muscles, etc. None of these things happen in isolation so really it's a good idea to have something to add in as you train if you're going to go for a prolonged workout. And chocolate milk is cheap, easy, and bloody effective in that respect.
Personally I tend to go for "no sugar added" powdered chocolate milk mix with 2% milk. There's enough "sugar" in the milk itself and I'm a bit sensitive to sugar as it is. So that is my compromise.