A very expensive 'piece of kit' (about GB£460) , but worth every penny if you are looking for the realism an MCP can give you in the 'Pit'.   I have it configured through the 737kbd file to command the PMDG 737 and it does it very well.
And the level of expansion it allows when used with the MCPEX1 and MIP737 boards and the CPFlight Radios is awesome.  I have almost all my systems working at the time of writing which makes the whole experience quite realistic.

Lets Take A Closer look at the CPFlight MCP and how it works with the PMDG
PMDG allow access to the MCP and EFIS controls by the use of simple keystrokes.  They also allow access to their product through several offsets via Pete Dowson's FSUIPC.  If you have the PMDG installed in your FS2004, you can assign keystrokes to a list of MCP/EFIS functions in the Keyboard Commands page under PMDG in the Menu Bar when FS is in windowed mode.   PMDG store these commands in a file called 737kbd.ini which you will find in the PMDG folder inside FS.  Now here's the clever bit, when you install the CPFlight software, you copy the 737kbd.ini file into the CPFlight software folder. Now CPFlight read that file and use it to communicate with the PMDG via their FS_COM utility because it knows what command to send for every function you have assigned a key to.   You have the choice of keys or you can use the recommended one's which CPFlight show on their website.

There Are Three Modes of Control
Ok so the engines are running and all systems are set and operational. You are flying the Aircraft 'Hands On'.  That is you have control of the Throttles and all Flight Control Surfaces. None of the Auto or Semi-Auto systems are engaged. 'You Have Control - It Is Your Aircraft'. The MCP is doing nothing at this stage.
Engaging the Autopilot, the Autothrottle and Flight Director sets the control system up for using the MCP in it's lower level of control 'Selected Mode'. Previously you had directional and speed control based on your inputs by way of the throttles and the Yoke/Rudders. Now with the MCP doing the work for you, command of speed,heading,altitude and vertical speed is controlled by dialling in the numbers and selecting which mode of control you want.
Finally, the highest level of control is achieved when LNAV and VNAV are brought into play.  You must have an active Flightplan entered into the CDU/FMC for these functions to be 'selectable'.  This is 'Managed Mode'.  That is the CDU/FMC is controlling every variable of the flight envelope from efficient power management thru navigating the aircraft in three dimensions and at given or computed  speeds right down to the touchdown zone on any runway at any airport it is instructed to do. AMAZING :o)
The CPFlight MCP emulates the BOEING B737 Mode Control Panel exactly.