Hey folks, can anyone out there tell me anything about the mid boost on older Jacksons? Particularly I'm looking at a Soloist built in 1990 with two singles and a humbucker. I don't think there are active electronics on board other than the boost. Does anyone have experience with this function? Is it good or bad?

They are pretty nice units, and dare I say essential with the original pickups. If it doesn't have the original pickups, I would just rip it out and throw it on ebay as it's probably not needed. Just make sure it works, as if it doesn't, a replacement is not worth what you'll end up paying, and the pickups will be kind of lame without it.