It's currently in a bugged state from what I can tell. I didn't know about this until a guildie told me, but you can basically spam any casttime spell and get optimal casting going. There's no timing involved.

What? Multiboxers don't use stopcasting? =)

I use stopcasting on most of my spells with the exception of the healer. Stopcasting to resheep, stopcasting to decurse etc etc. Kicks ass. (Don't worry, I get what yer saying Remote =)

I don't think it will stay like this. Blizzards original intention was that if you recast a spell before the cast had gone off, you'd start a new spell. Also there seems to be some issues with instacast spells like warlock dots (Ability not ready, unverified by me personally).