Cave Story+ excels in many of the major categories you'd rate it in. It's written with a lot of wit and charm. The gameplay is fun with a wide range of weapons that give you a lot of utility. Level design is varied and keeps things interesting and challenging. The soundtrack is a nostalgic trip into chiptune goodness.
It does have some flaws though. Its challenge mode is broken for me and many other players by crashing as soon as I reach the finish of the first challenge. Don't expect this to be patched as it's been known for a long time and nothing has been done about it. I'm also not impressed with its graphics. They're not terrible, in fact some of the character designs might be called cute, but many of the environments are drab mixes of grays and browns. I generally found the soundtrack catchy but there were a few tunes that became annoying.
Playing on "original" difficulty I thought that the combat had a satisfying level of challenge to it. However, the amount of damage you take from environmental dangers like spikes was too harsh in my view. There are numerous traps that will kill you in one shot no matter how much health you have. That is more unforgiving than I like, especially in a game with slightly floaty controls. Fortunately, the controls are consistent throughout the game so you can adapt to them and they become more manageable once you get the booster. Keeping with the old school theme you are unable to shoot diagonally, only in the four cardinal directions.
I'd like to close by bringing special attention to the rich world that is explored in Cave Story+. There's a fascinating backstory to discover as you play as well as fun characters to interact with. As with most "Metroidvania" games there are plenty of hidden and optional areas but here you won't just find power-ups. There are side quests, additional lore, optional weapons and gear, and even events that can lead to different endings for the game. This is a dream for players that love to dig into all the details of a game.