This is the Isomac Zaffiro with the back cover and the water tank removed:
 

 
Below you can see the PID controller prototype installed. A plastic box was used to protect from water splashing when refilling the water tank. At the bottom the solid state relay (SSR) is visible. The SSR makes it possible to control 240 VAC with the 5 VDC that the PID controller delivers.
 

 
LEDs are used to indicate the temperature. A numeric display was used initially, but was regarded too expensive and is also trickier to install in the espresso machine with a decent look. After using the LEDs for a while, I think they work very well. You can easily tell when the temperature is within +/- 0.5 °C, which IMHO is sufficient to achieve a consistent brewing result. This could obviously easily be changed to +/- 0.1 °C or whatever accuracy you want, but the temperature "window" that is visible would then also become narrower (if you don't introduce more LEDs).
 

 
Below is the final result of the prototype installation. On the low right you can see the 9-pin DSUB connector which is used for communication whith a PC. Through this it is possible to plot temperature curves, set the PID parameters or download new software versions.