I've played it for about 3 hours, and I'm greatly enjoying it. I understand some of the issues with the game though. Counter-attack is a cool move and gave the game a more action-based approach, but everything in the game charge at you, so at times it's probably a bit chaotic and unfair. But I'd love that they keep that mechanic, but they must perfect this. Aeion skills are cool, except for the scan skill. I think it's better to reward exploration to the players with a map station. Scan skill should be just to find weak points, because bombing randomly in previous Metroid games was tedious.
I love the art style and graphics of the game, and the soundtrack, something I always expect about a Metroid game. I'm playing it with headphones and I think it's the way to go. The combat is greatly improved, with free-aiming included but limited in a way that you have to stand still. Exploration and level design is pretty great. And, yeah, teleportation rooms are limited and I think that necessary to avoid a long backtracking, since the game is pretty huge. Surprisingly huge. Maps are really big.
I haven't had some boss fights atm, except for the Metroid ones. It's pretty satisfying to counter-attack them and bombard them with misiles, but it could be a bit repetitive in the end, unless they implement some new attacks and mechanics to the Metroids. But, yeah, it's a proper 2D Metroid game, and very good at that I must say.