Another drawback with the PMC series is the MO-1 headsets.  While this may not be an issue for some people, I found that not having the sound coming from both ears is annoying.  Purchasing the optional OCA-XO (open face) or OCA-XC (closed face) series of headsets helps with this, as the helmet then has two earpieces that balance out the overall sound although it is still monaural.

The headsets are connected via a coiled cable to the PMC. There is a quick disconnect point in the form of a "pig tail" that is approximately 3 inches from the bottom of your helmet once it is installed.

The coiled cable continues to a 1/8" jack for connection to the PMC main unit.  The cable also has a "Y" for connecting in a PTT switch.  The juncture of the "Y" is extremely fragile, and it is easy to accidentally pull these cables apart, rendering them useless.

I found that using the PTT that came with the PMC, rather than the one that came with the optional headset, is the better option - as it plugs directly into the PMC instead of being combined with the headset cable.

The optional headsets also come with a myriad of connecting cables, but there's no documentation to assist the new owner with understanding which cable serves which purpose.  After some experimentation, the three-ringed 1/8" jack with red tape on it works with handheld CB radios, such as the aforementioned Radio Shack unit.  The same cable without the red tape is for use in connecting the MRC-11X or auxiliary audio inputs.

The cell phone cable does work, although any called party tends to hear themselves echoed in the conversation.  The may be a fault of the cell phone, or it may be a fault in the design on the Nady device.  Regardless, I've been told it's rather annoying to receive a call from me on the motorcycle due to this echo. 

Bike to bike communications were tested with both the MRC-11X to another MRC-11X, and from CB to CB.  In both cases, the lower the overall speed, the clearer the communication.  None of the testing was conducted above 75 mph, and the average speed was 60 mph.

The MRC-11X is a combination FRS / GMRS radio with driver to passenger intercom capability and an AUX input for additional sources. 

The unit can be used in PTT mode or VOX activated for 2 way radio usage, but unlike the PMC-3X the driver to passenger communications is always on when the unit is in intercom mode.  I found this to be slightly annoying, because there was the constant sound of the background being amplified from the microphone and presented back in my headphones.

The AUX input provides additional flexibility, but no more than the PMC-3X and you've lost the FM tuner capability, unless the auxiliary device you are using contains one.  The interface for controlling the features on the MRC-11X requires some getting used to, and it isn't terribly intuitive.

The users' manual is an extremely valuable tool for navigating the otherwise cryptic menu system.  The MRC-11X comes with one Nady MMHS11XO open face headset.  Although it looks similar to the OCA-XO series of headsets, there's mixed results when using these headsets with the PMC series; the combination usually produces high-level feedback inside the helmet.

When used in combination, the PMC-3X with OCA-XO type headsets and the MRC-11X or handheld CB plugged into the unit in makes an affordable and safe combination for motorcycle communications, whether it's driver to passenger or bike to bike.

Bike to bike has much better results than driver to passenger, again based upon wind noise and road conditions. Experimentation is required to achieve the best results. 

Conclusion
Ultimately, you must live by the old adage of "you get what you pay for" when purchasing motorcycle communicators.  If you're on a limited budget and have patience, this might be an adequate solution for short distance riding.  If you're into distance riding or do not like experimentation, you might want to consider another option.