Mew2King likes it, Armada likes it. Most people seem pretty happy with it. Melee die-hards are going to keep playing Melee even if they pick up Ultimate as well. At the end of the day, Melee's competitive scene is built on game-engine exploits, and there will never be another Smash game quite like it as a result of that. That said, Ultimate has made some changes to it's engine to be more in-tune with the spirit of Melee, such as being able to cancel dash with any attack. That wasn't possible in Melee, but it provides similar options to wavedashing in that it makes movement itself less of a commitment. There's lower landing lag overall, lower than L-Cancelled melee moves in many scenarios. So the overall feel of the game is somewhere between S4 and Melee, but the controls and mechanics have been heavily simplified. Which even most melee players will admit is a positive thing.
Top tiers look like Pikachu, Bayonetta and maybe Pokemon Trainer, but Bayonetta is likely to receive significant nerfs before release. I also wouldn't be surprised if Captain Falcon is extremely good, if Ganondorf is fast-ish, then Falcon is going to be lightning. Ryu looks good, Cloud is still good but has definitely been nerfed. Ike's been buffed maybe just enough to break into high tier. Snake will probably be pretty good.
I could see Wolf being really good, but most people's impression of Fox is that he's awful here. It's not impossible for the spacies to be far apart on the tier list (look at Fox v Falco in SSB4), but I'm not sure yet.
Main concerns seem to be regarding the amount of hit-stun and the weird knockback physics.
