I am currently leveling my Hunter with my fiance. They are both sitting at level 62. I messed around with him a little bit to see how much Beastmastery was nerfed. They whacked us pretty good.
I am not going to go over all of the changes because you can just go anywhere and read the patch notes. I will give some comparisons before and after though.
First, before the patch as Beast mastery, I was doing close to 400 DPS on mobs running around outland. In a single target burst situation (such as the level 60 training dummies), I was doing around 500 DPS.
Now during my leveling, I am doing closer to 300 DPS. I decided to give Survival a go, and it had some very good abilities. I had a much higher DPS on the target dummy. However, in a soloing situation, my DPS was actually significantly less due to the fact that I had to lay off a bit on damage because my pet was not doing near as much. On the target dummies I was doing close to 500 DPS, but in Outlands questing I was only doing around 280 due to pulling back.
It is completly obvious that Beastmastery was nerfed A LOT. However, I believe it is still the most efficient way to level. You dont pull off of your pet near as much, and you pet is much tougher. Once you hit 80, survival will be the way to go. So, if you are still leveling, just stay BM. If you have hit 80, I would strongly suggest Survival as it has a very good damage potential as well as buffs for the raid.