To jump back a bit.. I use Y2K3 nearly all the time in multiplayer, me and a few freidns play a lot, mainly missions that who ever is hosting creates missions on spot.
It makes quickly making up new missions a lot easier, as you don't have to put all the heli dust scripts etc etc etc in, it's already there.
Only thing I've changed it replacing the Y2K3/Dta/anim.pbo with Sanctuary's Patrol animations (As the other people I play with have done too). You could remove the Y2K3 anim.pbo file and use the BIS ones if you wanted to, but Sanctuary's ones are the best sofar IMO
As for stabilty, I've have no crashes with Y2K3 in the last 6 months.. The only time it crashes (As with any decent sized missions), is occisonaly if I alt-tab out, it crashes (when I tab back in), this could be beacause I'm hosting a lot of the time..
I guess it's all down to personal prefernce though..