Protein Shakes as meal replacement.

I do this. Usually for my breakfast or morning snack because it's quick and easy and I workout in the morning so I'm ready for some protein replenishment by the time I'm heading out the door to work.

I make sure I eat every couple of hours, though, and it keeps my metabolism much more even.

My personal experience is that if I use MFP to log my entire day's food intake, I can just follow it and eat well, keep my sugar regulated, and keep my metabolism firing.

I think one a day is sufficient, maybe two if you're trying to bulk up, but nothing beats lots of good veggies and lean protein. If you are working for a specific goal, consulting a professional nutritionist might be the best bet to see if you can use the shakes to replace your meals. You may want to check and see how much sugar and other things you don't necessarily want are in the shakes, too, to help guide your decision.

Here's what most of my days are like and this works well for me--I am also someone that struggles to control food, so all these things are easy portions.

Morning: coffee, creamer, a little sugar, a Special K Protein Meal bar
Morning snack: Protein shake
Lunch: Turkey chili (or something similar I can make a lot in a crock pot and freeze into two-cup portions for work)
Afternoon Snack: Special K popcorn chips or Dannon Oikos yogurt
Dinner: Pasta with veggies and chicken or turkey or something similar. (Frozen Tilapia filets take a couple of minutes to fix in a skillet with some rice.)
Evening snack: Orville Redenbacher's 100-calorie single serve popcorn.

All of these things are great tasting, easy to make, and don't take a lot of planning other than at the store. Also, these are just my own things that work and mostly my opinions. ;-)

Good Luck!

Protein shakes don't generally have complete nutrition, so even with a vitamin, you would likely be missing out on very important nutrients. Your body can also only process so much protein at a time, so if you take too much at once, you are just wasting money on your powders. I occasionally replace a meal with a protein shake, but I also add psyllium husk and a powder called Amazing Grass to try to make it more complete. I only mix mine with water. I have done that twice a day, but I find real food to be more satisfying. You may lose weight doing that, and quickly, but it isn't sustainable. You can't keep doing that the rest of your life, and it won't teach you anything about healthy foods, portions, or how to cope with real life situations like going out to eat or holidays or birthday parties. Your money would be better spent meeting with a nutritionist and learning about how to eat the correct types of food in the correct portions, or get on one of those meal delivery services where you at least eat real food. If you are worried about portions, measure everything out and put it together the night before and pre enter it into MFP, so all you have to do is eat it the next day. I've done that before, and it is great, but I found it not sustainable. This time around, I am eye balling everything and entering as I eat. I go over sometimes, but I am learning as I go, and losing weight. I wish it would just fall off, but I didn't gain it all in a month, and I won't lose it all in a month.

Here is a handy portion guide I use for reference.

http://media-cache-ak0.pinimg.com/originals/9b/81/46/9b8146beca3d28ba897bb09b48dd0336.jpg

FIBRE

Oh man... you are going to be passing bricks.

If you're eating under your maintenance you will lose weight, but that doesn't necessarily mean it's healthy. Your body needs more than protein to function. Any diet you begin should include enough proteins, carbs, and fats for your body to work properly. As simply as possible, without enough of these nutrients your body will not work how it suppose to. The other issue is that multivitamins are not an appropriate substitute for whole foods when it comes to getting enough vitamins and minerals.

I'm doing the Herbalife shakes right now. I do two a day and most days I get a balanced meal for my third along with two snacks. I also take a multivitamin with every meal. Taking vitamins in smaller doses is easier on you, keeps them in your system. Drinking lots of water helps too. I'm finding that when I was mapping out my day with regular food I was losing weight better. I'm not done tweeking my plan yet plus I was on medical for 3 months and couldn't exercise. I LOVE the shakes! My coach did tell me that for the third meal you need to make it a balanced one no matter which meal it is . Later when you lose your weight you go down to one shake a day. It is a good idea to eat a balanced meal and MFP helps with the planning. I love this site. Good Luck.