Even as a relative newbie (I've played other tactical grid games), the combat is fun and engaging. Each unit has a wide variety of attacks and skills, and the situations each level and boss puts you in are fun and unique enough that I haven't gotten tired of it yet.

The story is also engaging, even if you've played the games. It's based on, because it puts its own unique twists on things in order to justify rolling up with a whole squad to beat Remilia. There are definitely surprises too, but they make sense, and the characters get a lot more, well, characterization than in the mainline games.

One of the biggest choices you have is at the very start, where you're asked to choose between Reimu and Marisa. The biggest change is at the start, but there are still route divisions with unique story. The groups come back together for the biggest fights, though. Most people recommend that you play Marisa's route first on Normal+ difficulty, then NG+ and play Reimu's route on Hard if you want to see everything.

Quick tip: You get, in addition to regular points for upgrades, EX Points which can be used to upgrade every stat. By contrast with regular points which should be spread around, you'll want to put all of the EX Points on a single unit. They will get special bonuses at EX 10. This purpose is more explicit in the older versions, where they were called Waifu Points. I've put all of mine on Shizuha, and she's startlingly powerful now.

FMW used to be four games, each covering roughly two main games. Even now, there's a sort of division between these acts. The Steam version has a shitload of bug and balance fixes, a full English translation pack, and better UI.

Anyway, I've been having a lot of fun with it, and if you're a Touhou or SRW fan, you should pick it up.