Whoops...thanks for correcting me, Willie-O...Martin Mandos are not nearly as valuable as Gibsons, though I'm not sure that makes them undervalued (how about "not overvalued"?). Your description of them is right on...they are very well made and nice looking instruments, but they just don't pack a punch...I think if Martin made better mandolins, they would not have stopped making them...
Bottom line in all of this is to play every mandolin you can get your hands on, figure out what you like, and buy the best one you can afford.