a. You mean eat nothin else all day but MRPs?
Or
b. Use MRP to replace, say for example, mid-morning meal?
selection a. would be idiotic. These are not designed for, nor can they be, a complete alternative to real food. Long term, you'd probabbly starve and wind up with all sorts of physiological problems. Short term, you'd loose strength, get sick, start voting republican or worse, etc.
selection b. is exactly what they are meant for. Food is always the best choice for a meal, but MRPs will work when you have exactly "2 minutes and 12 seconds" in which to prepare and consume your meal. They may often take the place of 1 or 2 out of 6 or 7 daily meals provided the rest is a good balance of actual food.
I use the GNC Mega MRP but there are better ones out there. I only use it once a day so I'm not overly picky about it. I hear the MetRX is good and of course the Myoplex (also heard it sucks shit now that it's been sold), couple of others too. If you use these MRPs correctly anyway, you're just as well with regular protien powder anyway since it's only 1 or 2 meals a day.