Wang, you're right, my S520EX is a 2002 model, it came with an Edge. Now, I didn't swap the Edge out because it was bad, it was jsut that the cosmo finish has rubbed off and it was rusting. I was upgrading my guitar so why stop there? Change the trem too lah!
I am seriously considering saving up for a chrome Edge again. USD240 methinks. OUCH.
Anyway, the ZR trem is good, apparently VERY good. Its got a built in tremsetter system (google this, more info online) that helps with return to zero point better at the cost of a stiffer trem and no more flutter. Its on a ball bearing system than knife edges so its smoother and less prone to wear.
It should keep its tunnig well, if you break the thin strings.
Double stops as in you play 2 notes on the guitar, and while holding down a note, do a bend with the other. On a normal floyd rose system, its possible to keep the 1st note in tune without bending it too (which FR users all do) and so the tremsetter in the ZR helps.
I'm in Jemsite, Shredcow too.
Well, do your research thoroughly... 1 man's comments don't mean much unless its Steve Vai telling you that the ZR is better than the Edge that he has been using all these years. Ask ppl here like Warlock, who owns the new Sabres with ZR trems.