The base game is good, but the best part is definitely the mods and custom engines. Psych Engine just improves the game so much with its QOL changes and amazing mod support.

I've seen people compare this to osu!mania, but mania doesn't have mods like FNF. There are lots of great mods that revamp the menus, add new cutscenes and even new game mechanics. I believe that's what makes FNF stand out the most.

I haven't interacted with the mod community though, so I have no comment on that.

It's fun, it can definitely use some more polishing though. Sometimes it feels unresponsive or not responsive enough, it lacks of basic modern rhythm game options such as custom maps, skins (or any type of mod loader in general), multiplayer and remaps for the keys. These types of games must be played with keys on a horizontal line because the way they're layed out on a normal keyboard makes it uncomfortable to play at times.

A lot of people love the presentation but I honestly hesitated to try it for that motive, I think the "quirky simplistic" artstyle has already died down for me.
Honestly, it's free, you can try it out and it's fun. I just hope it gets some more basic features because the week system gets old quickly. A multiplayer mode could really benefit from the "duel" type of rhythm game in which both sides are displayed.