I have never used a pack myself other than one I borrowed to try out for hiking and I found it hot there so I don't think I would ever want one for biking.

Something to remember is that unless you are doing long rides of over 2-3 hours you don't need to take that much water with you. The human body can only absorb about 750-1000 ml (3-4 cups) of water per hour so if you are not going to be out for more than 3 hours two bottles of water should be more than enough. The rest is just extra weight which you will be slugging around on your back making you thirsty but any extra you drink will just be sloshing around in your belly making you uncomfortable.