I never understood the hype about Vocaloid until I found the darker side of Vocaloid and found some cool songs. And I only listen to as many as 3-4 producers. Still never really like all the synthesized voices, but I got used to it because I really like the composition of some of the songs I listen to. Here are the three that I really like.
梨本うい (Nashimoto-Ui) Nico Nico Douga
That killer bass and loud punky guitars. This guy's got some amazing work. It seems that he formed a band and performed some of his vocaloid songs during live on vocals and guitars, but I don't know if they're an official band or anything. The band is called 'Araiyakashiko' btw. Anyway, some of his vocaloid stuff!
とある厨二 (Toaru tyuni) Nico Nico Douga
Again, a very talented person here. It'd be great if this person went on to form a band too~
マチゲリータ (Machigerita) Ohp
Machigerita has a spookier style, a bit halloween-y and trippy. He made vocaloid songs but he can sing too! And now he's doing solo work. It may interest people here that he did some vocaloid versions of visual kei songs (check second link in spoiler), but to me it's just kinda meh because of synthesized voices, haha. But it's still quite interesting to listen to.