Think he'll still have the advantage of range on a lot of characters. Other than that, can't tell anything yet without seeing him in action. But I still sort-of expect Marth to be better than average. Ever since his first appearance in Melee, Marth was alwas a strong opponent. He's just a very well written character. And I wish nearly half of the cast had a moveset as functional as Marth's.
Anything could happen though. Mario was also above average in Smash 64 and Melee, yet Bottom Tier in Brawl. Kirby was great in 64, terrible in Melee, and a little above average in Brawl. Marth could very well end up sucking if he has been struck with the Nerf Hammer. But still, I doubt he'd be anything lower than Mid-Tier myself.