I've been raiding as Beast Mastery since 5.0 went in, but with 5.1's last minute buff to Aspect of the Hawk and the change to allow Steady and Cobra Shots to be cast on the run, I went back to Marksman last night. The change is good! I'm posting decent numbers (for me) and finally not feeling like a deadweight.
I'm switching back and forth between Spirit Bond and Iron Hawk for soloing/raiding, as well as between Barrage and Glaive Toss. I shouldn't love Barrage as much as I do, given how much it tries to get me killed, but let's face it: 40 yard range, hits everything in front of me? It just makes some of the dailies go so much faster. On the other hand, I don't dare bring it into an instance, because I don't know what it's going to pull. (Also, randomly accidentally killing critters that may or may not be capturable battle pets... ;_;)
I'm not sure if I'm going to push to finish reputations first, to hit 150 mounts, or if I'm going to hit up the old raids for the battle pets contained therein, but so far 5.1 has been good for me.
PS: cloth heirloom panda is creepy: