Never really got into the vocaloid music scene. Triple Baka and PoPiPo and that's about it.
Kind of funny that people indirectly got into Vocaloid thanks to the popularity of Nyanyanyanyanyanyanya.
The songs are a reflection of their composers. Triple Baka and PoPiPo are general light-hearted pop fare, as are most vocaloid songs. However they number in the thousands and here and there you can find great stuff.
The diversity is huge too. There's a seemingly never-ending list of songs to sift through.
Sometimes I'll burn an empty Sunday digging through youtube, following links to stuff I've never heard before. After a couple years digging my vocaloid folder is at 75 tracks. So there's a large mediocrity to "decent" ratio.
If you limit yourself to just one vocaloid though, it'll get old fast. Even though there's no restrictions, most vocaloids seem to adopt a certain genre. Miku is heavily pop focused, IA has a strong dubstep current, there's more rock to be found amongst Lily, Luka, and GUMI...
Though they all have a thick nougaty pop center.
If anyone was looking for a jumping off point, search the vocaloid wiki for "Songs featuring ____". It'll list every song by a vocaloid that has significant views on youtube/niconico with links. For example..
Miku
Luka
GUMI
IA
And when you're done there you can join me and many others in lamenting the absence of more non-miku vocaloid figures.