I use it, and I've lost 11.4lbs in 7 days. It depends, if you're sensitive to whey it can stall you. Pay attention to the scale.
Although not purely protocol, I supplemented some of my P2 days with hemp protein (just hemp and fiber, NO sugar added). This helped on the days when I either needed a bit more protein or a change. In P3, I've been having some whey protein shakes either when I'm on the road or for a snack at home. Just be careful and watch for gains. I started on Jay Robb (expensive!), and switched to Body Fortress (usually about $15 at W-Mart, but scored a few on clearance for $11/2# tub). They mix easily and taste fine (which is totally personal preference, of course ). I don't know if it helped me lose, but it certainly has not made me gain, if that makes sense... Good luck!