I don't have 1st hand info on the CMMG conversion, but I have been doing some research on it over the past couple of days out of the same desire for a cheaper round. From what I can gather the CMMG option is not terribly accurate, and the best bet is to buy a dedicated .22lr upper...but they are expensive...ranging from near $400 (you can get a M&P 15-22 complete rifle for near that) to upwards of $900. I'd try and find a way to actually shoot a rifle with the CMMG conversion in it before committing to something like that, to make sure it's what you want, that it will give you the accuracy you want...otherwise, if it were my money, I'd probably just go with a 15-22 since the uppers alone are in that ballpark and up, pretty much, and it's got decent reviews for the most part.
*edit* Don't buy the 15-22 with the intention of putting a standard upper on it though, it will only take the 15-22 upper.

