I am of the opinion that Marth/Falco is largely stage dependant, with it going even on Marth's worse stages like Dreamland, maybe 55:45 Marth on most neutrals and 60:40 Marth on FD, as Snap said. It really comes down to the fact that Marth has so many options to deal with Falco's approach, and when he gets even one hit in, he has the capability to turn it into a stock. From a grab, everyone knows Marth is incredibly capable of destroying space animals, as M2K was once wont to do.
Falco, however, also has a good number of options, but really, Marth can deal with most if not all of them. Lasers prove to be little more than a nuisance as play level increases. Powershielding, for one, isn't overly hard to get, powershield->grab being a staple. Dash attack cuts under high lasers, and can also set up for combos.
Falco's shield pressure is a huge part of the matchup, of course. Pillaring can destroy a Marth who doesn't think through his options to it, since a pillar can lead to a poke which leads to Falco getting a hundred percent tacked onto Marth, and then a kill. That is Falco's power, in his comboing. However, one pretty simple option that works wonder to deal with it, in addition to choosing when to grab based on dair timing as is mentioned in the first post, is lightshielding. Lightshielding really messes with Falco's pillaring game, since it effectively pushes Marth out of the range of his shine with a light enough shield, leaving you capable of grabbing him out of it if your shield was the right size.
I'm sort of not sure why I typed this up, it just seemed like a good idea at the time I guess.