Not had a lot of experience of this system, but in absence of any other replies, a few thoughts.
My boat is based in Poole but as it's dry berthed I don't get through much anode material. If you get through anodes then Mercathode is probably worth it, and espcially so if you have a stainless prop. Have a look at the Mecury website, there's a bit in the FAQ's.
I know of people who have installed, it doesn't look too tricky. There's the control unit to mount in engine bay, an electrode to bolt to the drive (tailored for Alpha or Bravo drives). Electrical connection to battery, and from controller to electrode.
About all I can say really, except doesn't seem too pricey, can't remember price, but I do remember thinking 'That's not bad!'