From a game standpoint, he's pretty good. He has a good mix of useful specials, and some massive sword attacks that do high damage and launch. The trade off however, is his low weight and high windup and lag time. While I think he'll still be good, it might take a bit to learn him well. He'll probably end up high-mid/low-high tier.
But by God, at least for me there isn't another fighter that's just as fun to play. They really captured the spectacle and power of Sephiroth well here. From the Flares, which just create massive explosions, to his Thanos snap, to the way the screen darkens when he uses a powerful attack, he's an absolute blast to play. And there's nothing quite like having 8 Sephs go at it and destroying about 20 planets along the way.
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I still want My Good Flibbert and Trebbilf Doog Ym from Castlevania to be in Smash. [spoiler]HAIL HYDRA!!!!!!!!!!!!!!!!![/spoiler]