I lightened the flywheel on my (EFI) '06 S2RK last year at the same time I put in a new clutch and had the cam balanced. My mechanic had a flywheel that he'd machined down to a little over half the weight of stock, and he installed that for me.
Once in a while it'll stall when warming up whereas in the past I never had to touch it after hitting the starter button. Not a really big deal, just an occasional nuisance.
I like what it did to the feel of the throttle response. It's snappier, more responsive and it makes the bike even more fun to ride. When you roll the throttle off abruptly with a lightened flywheel, you get less of that Monster tendency to feel like you've dropped anchor, so it feels smoother.
I think it's a worthy mod for these bikes, but it's not as night and day a difference as some would have you believe.