I've occasionally used one, on longer rural rides in hot weather, when I know there won't be any place to refill my bottles or buy drinks.
The most important thing for me is to loosen the shoulder and sternum straps so that most of the weight rests on my hipbones, on the lower strap.
If you can find a shop where you can try on several models while sitting on your bike, that would be the best way. If you do that, bring something you can put in or strap onto the pack to simulate the weight of a full reservoir. I don't know much about cycling specific models, but I do know that hydration vests specific to running are MUCH better for running than the more widely available hiking packs, so I'd be looking hard at models designed specifically for cycling. Most of them will probably be tagged to mountain bikers, but unless you're riding in a super aggressive tuck, a mountain biker's pack/vest should still balance better on a road cyclist's body than a hiker's model.