I barely notice my Camelback when I'm mtbing. On the road, yes, because I'm in less of an upright position, but on the trails it doesn't bother me in the least. I'm sweaty enough as it is; the Camelback doesn't make me more so. It's so much easier to drink regularly and I have room to stuff food and other supplies. I use a women's version. I prefer one with both a chest and tummy strap.
ETA: This is the one I have. And it's on sale!

