If you are new to Hunter type, you should get to know the Aspect of the Viper at level 64, and it was a passive mana regeneration that didn't reduce your damage the way it does now. If you were in Aspect of the Viper, you certainly weren't getting the RAP bonus from Aspect of the Hawk. Despite this, there were a lot of fights in Burning Crusade that were pretty mana-intensive and you'd see a lot of endgame Hunters running around in Aspect of the Viper 99% of the time. But the point remains that Blizzard realized that Hunters were pretty mana-inefficient, spending all their time in Aspect of the Viper, and missing out on the yummy RAP bonus from Hawk.
   What's the new Aspect of the Viper do? It slices the damage you deal roughly in half but gives you mana back at a pretty solid rate. Some of this mana you gain passively, but a lot of it you gain back per shot. The damage penalty does not apply to your pet, but it does apply to things like Serpent Sting that you apply while under the effects of Aspect of the Viper.
   But when do you use it? At first, you can utilize it when you are out of mana. And you can use it when you are out leveling, soloing or mid fight in instance or raids with little or no mana replenishment from other classers or specs.
   Then you can use it between pulls. You pop into Hawk or Dragon-hawk, do a pull and look or you are almost out of mana. Right as the next pull starts, go back into Dragon-hawk. Now sometimes if you're in a really good group the tank is just be chain-pulling stuff left and right and you'll still find yourself having to either stop and drink, or Viper during the fight.
   Finally you can use it when you are briefly out of the fight. When you get the Grobbulus debuff and you have to run to the wall and you're much too far away to do any shooting, Viper until you can get back. When you've killed all the adds and are sitting around waiting for Noth to port back, use Viper.
   But vise versa, you can not use Viper when you are full of mana. This is a given that you don't want that damage reduction to be on you. It's easy to forget about it but once you've been playing with it for a while you start to remember it. There are addons out there that you can search for that will help remind you too.
Another case is that when you really need to kill something fast you can not use Viper. In fights where the DPS is really important and the Viper damage reduction gets really noticeable. In that case, if you run out of mana, it might be a good idea to use a mana pot rather than Viper. Remember that you can only use one mana pot per fight so time it wisely.
Once you squeeze out of Aspect of the Viper, there are a few things that can improve the performance. Aspect Mastery is a talent in the Beast Mastery tree, which reduces the damage penalty by 10%. This talent is a given for Beast Masters and several Marksman Hunters choose to take it too. There is also a glyph like Glyph of Aspect of the Viper increases the amount of mana gained on attack by 10%. Finally, the four-piece Hunter tier 7 set bonus will increase your ranged attack speed by 20% while you are in Viper.
It draws a conclusion that Aspect of the Viper has without doubt become an important role in playing a Hunter.