What Wyvern Sting does is sleep the target for 12 seconds and then drops a DOT (damage over time) effect on the target when it wakes up. The DOT does 900 damage for a level 60 Hunter.
It's actually a really nice talent, especially in a group setting. It's the Hunter's contribution to keeping a target out of the fight for a bit in the same way a Rogue saps and a Mage polymorphs. In solo encounters it is helpful also, as it lets the Hunter set up for an aimed shot that will awaken the target and start the 900 damage DOT ticking.
I need to play with this talent a bit more, but in the few days I tried it out there were a few things I didn't like about it.
1) The Hunter needs to be out of combat to use it, so any Hunter that gets attacked can't use this with any reliability. It's also on a timer, albeit a two minute timer.
2) The DOT does nice damage, but any other DOT from the same Hunter will overwrite the Wyvern Sting DOT. This effectively removes the Hunter's Serpent Sting DOT and the Viper Sting mana drain DOT from the Hunter's repertoire.
3) As mentioned previously, if I take Wyvern Sting I cannot take either Intimidation or Scattershot, the Hunter's only two interrupts and valuable abilities that let the Hunter disengage and get distance from a melee attacker to use his ranged attacks again. It's extremely important that Hunters have ways of keeping or getting space between them and the warriors and rogues that are trying to kill them.
In the end, I found that giving up Scattershot for an ability I can only use out of combat just wasn't worth it.