For an individually published series, Touhou has amassed an incredibly large amount of popularity. It's most so in Japan, of course, but more and more, the series has been coming into western audiences. Derivatives definitely help with this, with popular videos such as "Bad Apple" that spread about. Various memes have spawned from the series, with pictures such as the above "Touhou Hijack lol" (which might be replaced with another sketch of a character, perhaps with the word Touhou replaced with a word relating to the character), videos such as "Marisa Stole the Precious Thing", and even simple phrases such as "Sanae is a good girl."

As for a personal note on the series, I haven't met anyone who's tried the actual games and didn't like them.