Dovico has a nice paper and brief introduction to CMMI with a pretty straightforward conclusion: Is the CMMI right for you?
Unless you believe that common sense does not apply to your context, then the CMMI is a powerful tool to guide your process improvements efforts.
Studies have shown that companies that invest 5% to 10% of their operating costs into process improvement typically experience a return on investment of 100% the first year and upwards of 400% after 3 to 5 years. These returns on investments are based on reductions in the number of defects, faster time to market, improvements in estimation capabilities and better project control resulting in fewer schedule and cost overruns.
Unfortunately there is no reference to these studies, but let's just believe it.