C. Falcon has amazing speed, can combo reasonably well and is a dangerous character in many ways. Uair juggling, dash dancing, knee of justice and such are all moves you need to be very wary. But he still suffers badly from his old problems. That means our beloved Falchion utterly destroys Falcon. Jab, Ftilt, Fair, Nair, Bairm Fsmash and others, they all have better priority and range. Watch your spacing, keep an eye on his DD mixups and you'll be racking damage very quickly. Most of Marth's attacks can nullify C. Falcon extremely well.
Uthrow can lead into Utilt+Fair combos and set up for some **** nice edgeguard, same goes to upwards DB. Talking about edgeguard, it's your main way of killing in this particular match. You have solid answers to everything Falcon can do, your Fair goes through EVERY aerial he's able to do, with increased gravity, air dodge is too dangerous in most cases because Brawl+ increased gravity and C. Falcon is a fastfaller. A Dair or Fair can easily land a hit on his upB, sealing his fate. Fsmash also works wonders, it's pure ****.
About the cons in the matchup, I don't feel like listing them in details, but it's basically the same as always when playing against C. Falcon.
He's WAY TOO FAST
Has many deadly combos
Great dash dancing
Marth completely outspaces him and kills Falcon very easily. On the other side Captain Falcon is deadly fast and has nasty combos. 65:35 imo, Marth has the control of this match, but things may get tricky if you fall too much for his mindgames. Play smart and our sophisticated womanliness will surpass his gross manliness.