This is based on the two systems i've used over a long period, the one in my Clio and the one in my Passat.
I found the ones on the clio would go spastic fast for no reason, when it stoped raining they wouldn't shut off all the time.
Ones in the Passat are slightly better, not only do they relate to the amount of rain but also to the speed the car is going, but even at the lowest sensitivity setting i think they wipe too much / step up to a high speed too fast, oh and mine have that stupid 'have to activate the system each time you start the car'.
To be 100% honest the rain sensor in my Passat hasn't been the same since the windscreen was replaced but the current issues still existed just to a slightly lesser extent.
I think it's pretty common for a windscreen replacement to mess these things up, presumably something isnt lined up properly.
I've never had them (although I test drove a Mondeo Ghia with them and they seemed OK), but friends who have had them in Pugs or Renaults have thought they were useless, friends with VWs or Audis have been pretty pleased.