My Camelbak Blowfish is about 10 years old and I don't really notice the extra weight when I'm riding with it, since it's pretty low profile and stays close to my body. I've even replaced the original reservoir that came with it with a larger one. My back gets a little moist on longer rides, but not uncomfortably so. And I like that the water in the reservoir stays kinda cool. And it's usually pretty light by the time I finish my ride.

I've seen tube extensions for both Camelbak and I think Platypus might make them, too. My question would be if you strapped the reservoir to your rear rack, where would you clip your bite valve? I like that my bite valve is attached to my shoulder strap -- it's RIGHT THERE when I want to take a sip.

If you know someone who has one, why not ask if you can borrow to try it out before spending the money? See how it feels to wear it with water, on your bike, etc.