Nearing the end and think this one will easily be my GOTY at this point, having played most things I'm interested in till now and knowing there's not much left to come out this year, maybe AOE4.
I'll start with my couple complaints:
1) The music. This isn't that big of a complaint, the music is great...captures the mood, provides the ambience, etc. It's just not quite Metroid level good. There's not Phendrana Drifts, Brinstar, etc. in this one.
2) The games a bit too good at directing you where to go next. I keep reading people online getting lost and I just don't see it. For a Metroid game there's been very little opening the map and trying to figure out where to go next. Every time I get a new item I seem to be end up right beside where I need to use it and continue progressing, not to mention the game straight up blocking off old areas temporarily (thermal redirecting, destructible stuff, etc). That said, this also contributes directly to my first good point:
Good
1) In a world where games are full of bloat and trying to be 20-30 hour experiences despite only having maybe 6 hours of worthwhile content, this game does the exact opposite. Such a refreshingly concise and compact experience. One of the only games in recent memory where I just sat down and went through it in 3 decent sittings. No lulls to speak of, just an action packed game beginning to end. Yet the game still feels damn big. I look at the map and can't believe how much I've covered in a seemingly short amount of time. I wish more games would do this, I don't need everything to be a 20-30 hour experience and most games straight up just aren't good enough to try to be that.
2) Movement/controls are fantastic. Outside of a couple abilities which are giving my old man muscle memory fits it all just works so well and moving through the world feels fantastic. I can't wait to go back through old areas with my full kit and just collect items. You can tell they were definitely running out of buttons a bit by the end but (on a pro controller at least) it still feels great to me. They did a good job of compounding/replacing old abilities to compensate. The way they handle the missiles especially.
3) Looks fantastic. The first area was a tad bland but once they got into the more traditional Metroid biomes it really started to impress.
Overall just a fantastic game and a nice surprise for me. Between them even announcing/making this game and the unexpected level of quality given recent things like Other M I couldn't be happier. Hope this game does great numbers and they let Mercury Stream continue with the series, having a consistent 2D Metroid release schedule would be such a big thing for Nintendo's library going forward.
Now back to hoping for Prime Trilogy (or just the first remade, that works) and Prime 4.