I’ve personally been to 3 different Hatsune Miku concerts. They were all great.
But I think what a lot of people who only know of the concerts don’t get about the whole Vocaloid phenomenon is how grassroots it really is. See Hatsuki Miku was originally nothing more than a piece of voice synthesis software making musical vocals. Because the Vocaloid software is based around phoneme sample of real people each version has a unique voice and Miku’s creators decided to market a character to go with that voice.
Nobody ever expected her to get as popular as she has. Nobody planned to do concerts and stuff from the start. But people really liked the software and started making songs and then sharing them online, and this was actually encouraged. Eventually somebody said hey, since this stuff is so popular why not put together a concert. But even in the concerts the vast majority of songs were written by ordinary users and used with their permission. Heck, even most of the costumes are based on fanart and stuff. So it’s really cool to see it all come together like that.