I have about 1000km on Retrofitzed shoes. Mine are from DIY kits. If you choose the right shoe, that thing works perfectly. Simple to implement (but you need a brain to make that operation). It puts the stiffness right where it is needed most. Light and effective design. At the office you will forget that you are on SPD shoes.
I suggest to build your first setup from some old shoes. You will learn what to do, and you next setup will be better.