I held a Smashfest at my house tonight, and my Marth was playing very well. I did notice a few things, however.
After playing Kirk's Snake, I'm inclined to view the matchup as 70-30 in Snake's favor...Yes, I truly believe it's that hard. I found myself not being able to do much against Snake, and getting Ftilted out of shield even when I space my Fairs perfectly. It's tough to kill Snake due to his weight, making Marth's killing abilities less useful, when compared to say Ike's or Snake's...Speaking of which, Marth dies much easier than heavy characters, and Snake's power seems to kill Marth of very quickly.
As far as camping goes, it's very tough to stop a campy Snake. With characters like DeDeDe/Toon Link, projectiles are easy to stop because you can swat them away. With Snake however, I could not find a way around C4s, Claymores and if played correctly, Grenades and Usmash even. It's very difficult to keep away from them, especially if the Snake is forcing you to approach.