Next to useless.
Steady Shot now has a 2.0 casting time, affected by Haste. (The tooltip does not reflect this) However, our GCD is not affected by haste. That means, once your Steady Shot casting time is < 1.5 seconds, haste only affects your Auto Shots. Which means instead of buffing your 2 primary DPS abilities, it'll only buff one. Straight up AP and Crit will buff both, therefore theoretically scaling better. (Although there may be some benefit to buffing your Auto Shot for on-hit procs such as Wild Quiver or Imp Hawk)
Assuming you have a 15% quiver, in addition to 20% haste from a Windfury Totem (Yes, it now benefits us!) you have a <1.5 second Steady Shot cast. This is NOT taking into account haste effects from gear, or talent points.
tl;dr: Don't stack haste.
Feel free to correct me if I'm wrong, but this is what I've gathered.