This spell is amazing and gives rangers some massive burst damage for very little mana (in comparison to our fire and ice nukes in the same era).
I am sitting around 55% crit chance with my bow, and with capped Trick Shot AA I get at least 1 double archery attacks in every Hail of Arrows. Current damage (in my sub-par gear) is about 4k per cast, and with some practice you can hit multiples quite easily. This will also proc Flaming Arrows, which crits for almost 6k in era.
Speaking of mana, its FAR more efficient than our nukes (and does twice the damage of both of them combined in one cast).
The stats below are with full focus effects from OoW.
Hail of Arrows:
Mana: 284
Cast Time: 1.65
Cooldown: 8sec
Avg Damage: 4,000
Frost Wind:
Mana: 299
Cast Time: 0.5
Cooldown: 30sec
Avg Damage 1200
Hearth Embers:
Mana: 226
Cast Time: 0.5
Cooldown: 30sec
Avg Damage: 1000
So if you are in a fast paced XP group, just using Hail of Arrows on CD is not only about twice the damage, but the cooldown is only 8 seconds! You can go a long time just casting this one spell in a grind group without going oom. This skill alone is 30% of my DPS.