I've used a hydration system -- not Camelbak and liked it. I added ice in it and it kept my back cooler in the high heat and humidity. I didn't notice the weight one way or the other.

I've done an MS150 and did not use it for that ride -- there are plenty of places to refill your bottles along the way -- I just take two water bottles and put water in one and Accelerade in the other.

Oh, and I just bought a 72oz light blue Camelback -- it's purty. I'm looking forward to giving it a try.