Steve Vai is my favourite guitarist in the 'virtuoso' bracket - echo what others before me have said. For me he's taken the art of guitar playing further than speed (and doughnut) merchants like Malmsteen.

I like that he's willing to experiment, and that he releases stuff he knows isn't commercial, but that his fans will dig. My only criticism of him would be that he goes to far for my taste with the 'guitar as religeon' sort of an angle - if you look on Vai.com it gets a bit evangelical sometimes - just my opinion...

On tone - one of my mates has a JEM, and when I had a go on it it seemed to create all these mad harmonics all on it's own. I'm not trying to suggest that SV just picks up a JEM and slaps it about and it's all luck, but those Evo pickups are amazing in vai-esque hi gain situations. The circular vibrato is kinda interesting as well...