Maybe you should just keep them all and try and get different sounds from each of them! That's always a good idea
I have too much gear to fit!
You might want to check out JemSite for the answer.

From what I hear (secondhand), the Jemini isn't that good, but if Vai's actually using it...
Want: EH Deluxe Memory Man, Warmoth Strat, Budda Superdrive 45
On the Ibanez website (and on Youtube) there's a vid where Vai demos the Jemini, it's pretty funny too. He uses the boost side on top of his Legacy distortion, so you can't really tell what it really sounds like. The other side however, he uses on the clean channel, so it's all pedal distortion, and he gets a perfect 80's metal tone out of it.
^Yeah that video is what made me wonder about it. I'd use one side as a boost anyway to my Carvin V3 gain, but I really liked how the other side sounded with the clean channel. I tried using the TS9 for gain on clean but it doesn't sound so great.
I think the BOSS-SD1 sound kills the Jemini by far - for what I've hard.

What I heard on that video wasn't impressive at all. It was a funny vid though :p
Ibanez s540 with gold Schaller bridge and OMGGOLD hardware

Ibanez RGR320 with Lo-Edge Pro bridge and scalloped fretboard

Lee Jackson XLSC - 500

Roland Microcube

Dunlop Jazz III black
The left side gives the rich Vai lead tone.
The right side seems to be the Taurus Bulba/Bad Horsie rhythm tone, but slightly fuzzier.

I'm very much interested as well. Is there a comprehensive review somewhere (even better if it had vids/sound clips)? The Ibanez one is Steve playing on his expensive setup; most people have neither the technique nor the gear that Steve has, so it'll give me a more realistic overview, hopefully.
Lost interest.
^I'm sure it will be unbiased. It's a site dedicated to Vai's guitar, but they're the Ibanez crowd that's hardest to please.