In effect there are some big differences between a JEM and an RG .
The first is the aesthetics. The JEMs all have a much bigger visual impact than most guitars, whether it's the industrial looking DBK with the industructable road surface finish, or the pimped up white and gold VWH, or the refined and elegant vintage looking VSBL. The monkey grip and Lions claw rout add to the image as well as having practical applications.
In terms of sound, the JEMs offer a different slant on tone compared to the RGs, they come stock with the superb DiMarzio pickups that Steve chipped in on the design of, and especially in the higher models have a range of woods that are currently not available on even a Prestige series RG, Alder bodies with a choice of Rosewood or Ebony fingerboards.
The necks themselves are a little thicker than most found on the RGs which not only helps the tone somewhat but for many of us increases the playing comfort, especially when playing a long gig or practice. They tend to have a smooth, light satin finish on them, not sticky like a painted neck, but a finish that still maintains a good feel of the bare wood whilst also offering it some hard protection, a feature common to many of the higher guitars in Ibanez's range.
Compared to other similar guitars, it's just a matter of personal taste. JEMs often get compared to the Jackson Soloist, a great guitar, but not one that has identical features. The JEM is obviously a bolt on design (as opposed to the Soloist's through-neck construction) which many find lacks sustain, but I prefer to describe it as addding sparkle to the sound, a trade off I'll make any day of the week.
The Ibanez guitars until recently came with the Lo-Pro Edge trem, I'd consider this the most stable and comfortable design inthe market place. This changed a few days back and the 2003 line will come with a redesigned and publically unproven trem.
I choose the JEM because when I play that guitar, I'm not limited by my instrument in any way, sonically or physically. I know that if I can't play what I want, how I want on a JEM, there's no guitar that I can do it on.
Yes I've tried any alternative I can find, and I've always maintained I'll switch brands if and when I find something better. I still haven't found the better yet, and I've been playing Ibanez guitars pretty much exclusively for the last 10 or so years.