Overall this game is clearly the summation of every previous GB Mega Man game, and it has the quality to match. Mega Man's physics and controls feel just as tight as always, but the replacement of the standard charge shot with the new Mega Arm really changes things by a surprising amount. The Mega Arm is powerful, but when it's detached you can't fire for a solid second, which may not seem like much, but Mega Man is a game where you tend to be bombarded by bullets every second. That second of time where you can't defend yourself really does matter, and it forces you to consider "Should I use the Mega Arm and hope it's strong enough or be safer and just use standard shots?" I really like this dynamic overall. It shakes things up, but it's not so different that it's alienating to long time fans. The Mega Arm's upgrades are also really useful and fun to mess around with, like the grab upgrade that allows you to pull enemies towards you.
The game's difficulty balance is great too. It's always surprising you with new enemies and mechanics, but every enemy's weakness is learnable and exploitable. The game is always presenting you with new challenges, but they never feel insurmountable, so you're always kept on your toes and struggling but get through once you figure things out. This is how difficulty in Mega Man really should be I think, fairly tough but not so frustrating. I appreciate the game's liberal use of spikes too, they're not practically omnipresent but they're there where it counts. Only complaint I have here really is that the game sometimes likes to put spikes in sections where you're falling down and have no possible way of really seeing them coming, but even that wasn't super annoying for me.
Levels themselves are pretty cool too. They have a lot of interesting themes, being a game revolved around space of course. I like how Jupiter's level just takes place on the surface of Jupiter, a straight up cosmic level is pretty unique in the wider context of Mega Man. The levels have some pretty cool gimmicks too, one I particularly like is in Neptune's level. There are these torrents of water that push you back emerging from pipes, but what's cool is that some of the pipes have cracks in them and shooting the cracks releases the water which can potentially push you into a bottomless pit. It forces you to think carefully and not just fire randomly with no precision. All the stages don't overstay their welcome too which is nice. Also shoutouts to that one part near the end where out of nowhere it becomes a bullet hell shooter for a few minutes, that's pretty cool.
Unlike the previous GB games, this game has a pretty much entirely original cast of enemies with their own little gimmicks attached to them. For example, there are these crab enemies that charge up bubble balls that trap you if you get touched by them but can be broken with enough shots, so you have to switch back and forth between shooting the crab and shooting the bubble. It's honestly fairly minor but I like those sorts of enemies. Some enemies go even beyond that, like this one in Saturn's stage that straight up slows time if you get to close. Can't think of many standard enemies in the classic series that really mess with the game itself like that. I hope we get to see some of these return in a future game, they're fairly interesting overall and could be cool to see in 3d.
The bosses really shine imo. Every single one of them is fun to fight. Their difficulty feels perfect, they're all decently tough, but their patterns can be learned while at the same time they're attacks keep you on your toes. One I thought was particularly cool is Mercury. He seems fairly standard, attacking by splitting up into orbs and firing shots, sorta like a Devil monster, but his gimmick is that if you get hit by one of his shots, he steals an item from you like an energy/weapons tank or some currency. That's entirely original for a boss in the franchise and adds extra incentive for not just tanking his attacks, in a way teaching you how to dodge boss attacks. Feels organic. The weapons are pretty great too, they all have an original function and use, and some like the Bubble Bomb or the Grab Buster I found myself using quite a lot throughout my play-through. Props to this game for being one of the few classic Mega Man games to not have Wily as the final boss too, Sunstar is a really cool and climactic fight.
Overall this is one of the finest classic Mega Man experiences on this here Earth (or should I say Terra?). It easily holds up among the greats like 9 and is a fresh, fun experience. Whether you're a Mega Man fan already or haven't touched the franchise much, I'd definitely recommend this game. It's not that much of a time-sink but what is there is just plain good.