Fully resisted by all bossed from HM Titan onward. There's always room for SE to adjust this, but as is, even if a boss is ROOTED, they can still freely TURN and attack anyone, so even if the root currently works on some bosses, it doesn't stop them from turning like it does with you.
@Voyage: Exactly. Both tanks will be incredibly useful, yet have superior individual tanking situations. Y'know... the way the game should have been from the start. I know I can't wait to start tearing shit up with my Coil Pld when these changes arrive.