MM is a really great game, but it's value is more due to the fact that it is still a route monster than it's true play value. If you want to pry them out of the hands of ops, you have to pay.
While it might be in my top 10 or so games, I'd rather have two or three of the next 30 or so games that I like than having one MM, so at no point in my collecting days has it ever made sense to pick one up. 10 years ago, they were $4k machines when you could pick up LOTS of stuff for $800. It made the decision easy - I could get five games I really liked instead of it.
Prices are up a little across the board now, but the five for one still holds true. In fact, I could buy my favorite six pins for going market rate, and still be WELL under the $13k or so that MMs get listed for often now. Makes it really hard to justify one.