Easy to install - 15 minutes maximum. Only potential difficulty is the tiny space in which to get at the single screw securing the switch to the body of the rest of the unit. Easily solved by using a spanner and a hexagonal Phillips screwdriver bit
Resolved starter motor problems - no more random clicking, and engine now starts first time, every time.
Also windscreen wipers now appear to be working a little faster.
I don't know if it's my imagination (probably is) but the engine also appears to be running smoother and more efficiently.
Not exactly cheap, though, for what it actually is.