Almost always on the mountain bike. Almost never on the road bike. When riding singletrack, there are very few times when I can safely take a hand off the grip long enough to grab a bottle, sip and replace the bottle. No such problem on the road. As long as I can hydrate without the load on my back, I let the horse carry the load. The only time I use a hydration pack on the road is when I know I'll be riding somewhere where there are no opportunities to refill bottles after a couple of hours or so. Even then I prefer to just carry an extra water bottle in my jersey pocket or on the rack or saddle bag if I'm on a bike so equipped.