While I've never played any of the Touhou games, from what I see it's pretty infamous in other Danmaku communities due to how...I dunno, overblown it is and its apparent lack of difficulty compared to other Danmaku (again, from what I see, not from what I've played).
Looking up fan art on any -booru site for anything at all will most likely yield a Touhou crossover at some point.
I do like Flandre's theme because of the RAN-RAN-RU remix.