Beat the game 3 days ago, it is a solid 8 for me. I enjoyed it a lot and it was challenging just the right amount.
All the boss fights were great, hard but after a few tries and learning the pattern you can do a no damage run even if you are not the most skilled player.
Last boss wasn't hard at all. Gameplay was the best part of the game, I am someone who enjoys games with story, but I know that isn't the goal with a game like Metroid. If the game had a story that captured the gamer even more could be a 9 or even higher.