ROB is a real pain to fight, just because of the nature of the match-up.
On the ground, ROB's F-tilt is a big pain, but like Shaya said, we just wait for the whiff or shield it. D-Smash is lolwut easy to DI out of, other than that ROB doesn't really have a ton on the ground, besides his camping.
On his camping, Marth doesn't have as much trouble as others simply based on his speed. Gyro and Charged Lazer should be power-shielded on reaction, and weak laser is a nuisance at best.
In the air, ROB gets ***** pretty hard once Marth is in control. Like Shaya and Stingers mentioned, uair ***** hard. Fair doesn't **** as hard as usual, since I'm pretty sure your fair might beat out ours slightly, so it's usually safest to fight ROB from below. All we really have to do is watch out for Nair and Bair tricks.
IMO, this match-up really comes down to who can get the other offstage. ROB has some insane gimps thanks to his recovery from forever, but Marth has a pretty easy time guarding ROB's super predictable recovery. Recovering low is risking getting Dolphin Slashed, unless the stage is deep enough that you can go under both Dolphin Slash AND the stage, and still recover, and going high will put you in a position to be put back out by bair, or juggled by uair.
ROB really relies on pestering Marth and disrupting spacing, then going for the gimp.
I'm also going to have to agree that this is probably one of the most even match-ups in Brawl.
50:50.