I fitted one in mine, and I have to say you are all rather negative about it, quite unfairly..
Mine had a Live, Neutral and earth taken from a junction box in the loft to power the unit, then the existing 2 switch wires were fitted across the relay as normal.
I have to say if it's set up right, it's lovely..
I set mine for a 10 minute timeout, and I've had 1 darkness moment, fixed by sticking a finger out of the shower curtain and making a gesture at it.
I bought it for the sake of being able to run and hurl without missing (missus and morning sickness), and for little person#1 to be able to walk in and use the potty/toilet without worrying about a lightswitch.
Light fitting #1 has a halogen "lower energy" 40watt equivalent in it for instant light, fitting #2 has a CFL that brings the lights up to full level without costing the earth.
Still my favourite modification to the house.