Legend has it that M&M's were first popularized by giving them to World War II infantry soldiers in the field, as a convenient snack that wouldn't melt easily or get chocolate all over everything. Encasing it in its famous candy shell ensured it "melts in your mouth, not in your hand", as the slogan goes (unfortunately the candy shell does get a sticky, colorful film on your fingers in extreme conditions, but the chocolate does, indeed, behave itself). Had Peanut Butter M&M's been available at the time (they were invented in 1990), I assure you the D-Day landing at Normandy would have simply steamrolled over the Axis forces on an unstoppable juggernaut charge to Berlin.
Peanut Butter M&M's aren't just candy, they're food. This is how they stack up to puny plain M&M's, based on a standard 1/4 cup serving size:
PB Plain
Calories: 230 vs 210
(from fat): 110 vs 80
Fat: 12g vs 9g
Protein: 4g vs 2g
These things and a few tall cups of coffee can keep me going for days with no supplemental nutrition. The Surgeon General does not recommend this diet, and in fact would probably have me beaten in a back alley someplace if he knew about this.
Peanut Butter M&M's in some respects are similar to the inferior Reese's Pieces, introduced 10 years earlier. I don't know who Reese is, but his pieces failed in two respects. First, they have far less peanut butter per candy. Peanut Butter M&M's look like plump, succulent M&M's just bursting with peanut buttery goodness, while Reese's Pieces are about the same size as plain M&M's and do not provide the optimum ratio of candy shell to peanut butter filling. Second, Reese's Pieces do not contain any chocolate. Peanut Butter M&M's, however, have a thin, delicate film of chocolate between the peanut butter and the candy shell. Just enough to add a bit of flavor without distracting you from the sheer flavor power of the pure peanut buttery core.
For some reason, I've gotten Peanut Butter M&M's in two different shapes. Some bags have candies that look like plump Plain M&M's, and others look like ever so slightly larger than normal Peanut M&M's. Aside from the geometry they appear to be the same. I can only assume Mars has two factories pumping these out using machines that are designed somewhat differently.
Peanut Butter M&M's are represented by a female anthropomorphic green M&M in the commercials, voiced by Cree Summer. Rumor has it the green M&M is female because the green ones were once rumored to be an aphrodisiac.