This is an age old question, the answer is pretty much what Marcel has said, it all revolves around the trem! The older JEM 555s had a trem (I want to say Lot TRS 2 or something?) that wouldn't stay in tune. Once that was replaced e.g. with a Gotoh, Schaller or real Floyd, then the tuning was spot on.
Feel and sound wise, I personally can't fault them
I don't like the half vine, but I love the Dimarzio Evos in basswood with the little bit thicker neck than the normal RG.