I heard about Gackt after playing Dirge of Cerberus. He actually did the two songs in DoC, and yeah, Genesis is modeled after him and I think he's also his voice actor in the Japanese version of DoC and Crisis Core.
I don't agree with the comparing him to Michael Jackson, and all reasons have been stated above by others.
I know he's popular for the guy-on-guy fan service element though.
Probably the most famous example would be his live performance of the song "Vanilla" where he did all sorts of things with the band members on stage.
If you're not grossed out by that, here's a video of the song in concert
http://www.youtube.com/watch?v=JIl85cKtSDg
(Note how the the fans, mostly made up of women, go crazy

)
And the song itself insinuates something along the lines of yaoi, although the original music video doesn't seem like it:
http://www.youtube.com/watch?v=FNEUN...eature=related
And on top of that, he supposedly said in a interview that he doesn't care about gender when it comes to love, and he supposedly has a special relationship with another J-Rocker named "Hyde" and fans of course take it to the next level.
1) This is a photo-shopped image, where Gackt is originally kissing a girl, but add Hyde's wing tattoos, and who can tell the difference?
2) Here's a poster of the vampire movie they did together "Moon Child"
3) And here's them just hanging out
But anyways, Gackt has a great voice (even if I don't understand what he's singing) and very nice for the eyes
