Vai's idea was to come up with a pickup that is tight in the bottom end, and with a smooth top end. This allows him to crank his amps a little bit more.
I put these pickups in my no-longer-available Anthem guitar, which is a shredder type of guitar, with a Floyd Rose. I then tested with my Marshall JVM410H and my Line 6 POD HD500. The results brought a smile to my face.
The neck humbucker is fat but also articulate. It's very sweet sounding. I get clarity even with a lot of distortion. Not muddy like some neck pickups are, when using a lot of distortion.
The bridge pickup has more midgrange focus, and this is something I really like. It makes your guitar tone stand out more, and you will hear yourself better in band situation. I also find that with my Marshall, I get old school Marshall tones easily, even though I am using modern gear, so to speak. Very rich and fat tone, without being of the very hot output kind. I also found that legato playing was very easy - the notes are clear; it was very easy to play fluid lines with hammer-ons and pull-offs, without working the left hand too hard.
In summary, I was pleasantly surprised by these pickups. I do not like edgy, bright distorted sounds, and sometimes, Vai has had that sound. I don't care much for that tone, but with these pickups, it really sounds old school. I was thinking old Van Halen tones, when I played through my Marshall. Fat and rich, without being muddy.
I give these pickups a 4.5 out of 5, for that old school rock tone.