There are several good threads on the chainsaw forum about these, going way back.
I currently have and use the first generation oregon battery saw, it's spiffy. It was gifted to me by oregon following an all day test I did with one of their reps.
They now have their second generation saw out, got a chance to run one two weeks ago, it is much better for torque and chainspeed, they switched to a "brushless" motor. The oregon also has built in powersharp, which is really cool, dull to sharp in five seconds! With the addition of several other battery tools in their lineup now, I would say that is one good option, but there are more.