+1 on leveling them as boomkins. When I leveled them, I kept them boomkin all the way to 80 and had very little problem clearing instances along the way. For boss fights, it was all about getting your dots on them and going to town, then whoever starts getting beat on, throw 5 regrowths. Once you get treants and startfall, things become a lot easier. I just started leveling mine again (currently 81) and still boomkin, however I'm in the process of collecting all feral gear, so I'll be making the switch soon.

For the more challenging pulls in instances, you also need to remember that you have 5x entangling roots at your disposal. At early levels, it's going to be tougher regardless of the comp due to not having the deeper tiered talents or the HP to survive easily. All of the instance content up to and including bc can easily be done with 5x boomkins at the appropriate level.

I haven't done a whole lot on them since the expansion hit, but I did manage to give blackrock caverns a try with 5x boomkin @ 81, and was able to down the the first boss without losing anyone. I opened with treants and lots of dots, then once the chains appeared, i popped starfall which broke the chains almost instantly, giving me a lot of time to move away and dps freely on the boss. I don't expect a boomkin team to go much further, but for leveling, they are still very efficient.

If you must level them as feral, then I say just go 5xFeral as you'll have close to double dps which means stuff drops faster in addition the instant nature casts are great for topping off whoever is taking damage.