It always takes around 1.5-2 seconds to cast and it cools down in 15 seconds. It does double the damage of a normal shot and it never misses. It does interrupt your auto-attacks, not as much as other skills, but enough so I tend to only use it when I am not hasted and/or when my Rapid Shot is not active (e.g. during the time while Rapid Shot is cooling down). I sometimes use it as the first shot of a battle (my current built does not have Longshot) and I very often use it as the final shot; I have been using it long enough to gauge when one Doubleshot will finish off an enemy. It is has an advantage over other skills in that you only need to get it to 42 points to max it. I have intensively experimented with all the Ranger skills and it is one of the skills I feel is worth using.