Did you compare its speed to built in atan2 function?

I counted the cycles for you method and it comes to around 50 plus a conditional. Built in function should be around the same.

how's it compare to a CORDIC?

I wrote it for the Parallax Propeller, which is slow at doing divisions - so I tried to keep those to a minimum (one, which isn't too bad).

With a CORDIC the only divides should be by powers of 2 in the main loop and taking out the gain factor at the end, all of which can be turned into multiplies?

Thanks, I'm going to go back and try it :)