I had a M&G a few weeks ago and here's what I gleaned....

If everything goes well, it's a letter of rec on AirlineApps from the Chief Pilot. The original premise was that this rec would move you to a stack of 300-400 individuals with CP endorsements. Right now, it's apparently 3(ish) times that. I guess a lot of CP's are seeing some really good candidates. The particular domicile I visited allowed each pilot (Capt or FO) to push up one candidate for a M&G per year.

The experience was outstanding. Everyone was very welcoming. I felt like they were trying to sell the company to me, not the other way around. I wasn't too sure about the whole idea of dedicating two days of leave, airfare, and a hotel for a 45 minute meeting. In hindsight it was well worth it. I learned a lot, and got some good advice for the interview (fingers crossed). I equate it to a job fair, only you get to meet a CP and there aren't 300 other people dressed exactly like you.

I footnote all of this with YMMV. It appears each office does it differently. For better or worse, I think it's a great opportunity.