My experience has been like Billc's, both the 2001 e and the 2002 c worked as well as you could do by hand. If it stopped raining while the system was on, the wipers would no longer run at all.
Then last week my 2002 c began doing exactly what Clemson91 describes - when you first turn on the wiper the system is fine, ie you'll get one or two occasional wipes. Once the windshield has been rained on the wipers switch on permanently, and don't stop till you shut them off. Driving into the garage on a rainy day is a good test - they used to shut off, but not anymore.
The c goes in next Friday for its "A" service, and they will look at this also. I'll post the outcome...