The only Camelbak I've tried is the waistpack, Flashflo, which on the website says 1.3L but I swear the one I got said 1.8L. I wear it running and biking, and bought a clip to add on so I can clip it to my jersey.

I did not get a backpack mostly because I sweat so much running that an entire pack would be too much for me in the summer. I have used it several times riding, and it's great, though I'm pretty sure it covered up my jersey pockets. I carried gels, my cell phone, keys, a couple of other things. No huge cargo capacity, but I have a frame pump and bag on my bike for tubes and tools, too.

I had to buy the clip to put it closer to my mouth (otherwise I have to reach to my waist to get the valve), I believe the backpacks come with them. The capacity is perfect for me for about 2 hours of riding/running in the sun, much more if it's a cool/shady day or low intensity ride, less if I'm super thirsty or it's a warm/humid day.

As for weight, I didn't really notice it biking, but I notice it a lot running. I fasten it as tight as I can to my waist for running (or it's really bouncy), but I can wear it more loosely for riding. I think a backpack would be similar on that front, but a little better distributed. As your ride goes on and the camelbak empties, the weight becomes even less of an issue. I usually forget about it entirely within a few minutes.

Sometimes I put mine in the freezer 30 minutes before I'm going to go to make the water super cold. Having it against my body can make either my body colder or the water warmer. Sometimes, I empty a bottle of filtered water I always have in the fridge into the camelbak and restock the fridge. I would store mine in the fridge, but then it picks up the "Fridge Smell" if we have leftovers in there Sometimes water out of the tap is cold enough, but I find that when I'm exercising I like my water COLD, colder than I do otherwise.

I really prefer it to reaching for bottles for water, I can carry much more this way, and it means I can put cytomax or something in my polar bottle instead.