The biggest issue with the IMA is the fact that the CMA is not all that well known and marketed. Additionally, while we all may hate the AICPA at times, the level of organization and formality followed for the CPA designation is simply exceptional when compared with the IMA. On paper they have a pretty good program, but because of their size, or rather lack thereof, they simply don't have the man power to compete.
I did the CMA tests first because I qualified to take them, while I was still short on on credits for the CPA. Having a good idea on the content of the CPA now, I will say that the CMA is a 100 times more applicable to what I do (upper management for companies in the private sector). BEC doesn't really begin to touch the depth of the material that is covered in the CMA as it relates to costing, manufacturing, etc. As I was studying for my CMA I actually learned stuff that I still use every single day- I loved it. I can't say that I will feel the same way about the REG or AUD sections of the CPA.
I've worried a little about the CGMA designation, but the truth is that as of right now not very many people know what it is, especially in my field. Most of the time when the CMA has come up people do know what the CMA is. If they don't its easy enough to explain. At least with the CMA its possible to say “Its a designation which required me to pass two pretty difficult tests and requires I maintain a certain number of CPEs.” Vs: “Oh its something I paid for because I'm already a CPA”.