The protein helps *some* people feel full for hours. Right now, I'm actively logging food in My Fitness Pal. I've lost 9 lbs this month. I am participating and reading their forums a LOT. They have a very active, knowledgeable forum. I was doing tons of reading on the shake replacements (which many people do there) and it seems that there is a cadre of people who don't do well with liquid meal replacements; specifically, no matter what, some people just don't feel full/satisfied on them yet they have used 300 calories. It's really trial and error to see if they work for you or not in that way.
I personally know someone who used shakes twice a day just for discipline. She was out of control with food (lots of foods caused triggers of overeating) so the shakes for two meals got her under control. It's not something you can do forever, but if you are looking for the discipline and "quick start" to weight loss, they can work in that way.
OP, since you mentioned jump starting your metabolism, I'm assuming you are looking for weight loss.