In the club rides I have been on there are always 1 or 2 folks with a hydration pack, maybe 5%. The one I bought was a nalgene 2L, red to match the bike.
This may open a can of worms but ... One thing I learned from sailing though is that active weight is better than dead weight. Water bottles on the bike are dead weight, weight on me (the pack) is active. Biking is not sailing but I can swing my weight into action standing up out of the saddle or I can let it sit. Yes - I have to pull it up a hill no matter what but I suspect that a pound on my back is better than a pound sitting on the bike.
My plan is to use the pack this fall if I start out with a jacket or ride any significant distance. I will use it in the century next weekend but make sure I don't tie up the line filling it (good advice on that point). A 2L load should last me 45 miles.