I use them together and find that they compliment each other perfectly, thanks to an obvious similarity in tone; and yet, each instrument has much to distinguish it from the other. I would never think of using one to fill the other's place, even though it's possible in some instances. The Poly Evolver Keyboard has advantages and strengths, but so, to, does the Prophet '08. For example, the P'08 has a better sawtooth solo sound, whereas, the PEK's version of the same sounds a tad too thin. But then, the PEK offers many swirling ethereal digital pads that the P'08 cannot even approximate.