It is a casting ability, suffering from delays by enemy attacks.
Tips and tactics
To understand how steady shot is affected by haste ratings it's important to understand how haste is calculated. A haste bonus is multiplied by frequency to achieve post-haste speed. World of Warcraft expresses all speeds as period rather than frequency, so the period must be divided by 1+bonus.
As for example in the above case:
Steady Shot Cast Time = 2 / 1.15 (hunter base ranged haste) / 1.2 (Serpent's Swiftness) = 1.4493
1.4493 + 0.5 + 0.2 = 2.149
2.149 x 1.15 x 1.2 = 2.966
Steady Shot is affected by haste bonuses and ranged attack speed bonuses including Rapid Fire and Bloodlust.
Hunters are the only class whose GCD is unaffected by haste.
Patch changes
Patch 5.2.0 (5-Mar-2013): Steady Shot now deals 20% more damage.
Patch 5.1.0 (27-Nov-2012): Steady Shot, Cobra Shot, and Barrage can now always be cast on the move.
Patch 4.1.0 (26-Apr-2011): Aimed Shot and Steady Shot should no longer start casting Auto Shot on a new target when the "Stop Auto Attack" option enabled.
/Patch 4.0.1 (12-Oct-2010): Available at level 3 for 57. It generates 9 Focus.
Patch 3.1.0 (14-Apr-2009): Now does correct damage with heirloom weapons.
Patch 3.0.8 (2009-01-20): Now gains 10% of attack power as damage instead of 20%. Arcane Shot: Mana cost lowered to match the cost of Steady Shot.
/Patch 3.0.2 (14-Oct-2008): Now uses ammo. In result, its bonus damage has been slightly reduced. Players can notice a damage increase based upon what ammo they use. Now has a 2.0 cast time, up from 1.5. No longer clips Auto Shot making weapon speed less relevant. Steady Shot is available beginning at level 50 instead of 62.
Patch 2.4.0 (25-Mar-2008): Hunters will no longer spin around if they cast Aimed Shot or Steady Shot while facing away from their target.
Patch 2.3.0 (13-Nov-2007): Tooltip clarified to indicate base weapon damage is used in the damage calculation.
Patch 2.2.0 (25-Sep-2007): The tooltip stating the percentage of Attack Power gained by Steady Shot was inaccurate and has been corrected. The damage remains unchanged.
Patch 2.0.6 (23-Jan-2007): The base damage for Arcane Shot has been reduced by about 9% and the bonus damage from ranged attack power reduced from 20% to 15%. This patch made Arcane Shot and Steady Shot do roughly the same amount of damage.
Patch 1.5.0 (2005-06-07): Special ability shots no longer add the ranged weapon speed or take into account ranged weapon haste when determining the actual cooldown. The tooltip and cooldown timers should now properly reflect the actual cooldown.