To say that DM courses are not a "real challenge" is really not fair... in my opinion. There are a lot of quality DM's that work their asses off and are extremely competent.
I can say with certainty that the NAUI Rescue and DM courses that I took in the mid 90's were very challenging both physically, and academically. Long, cold and dark days up here in the Pacific NW with little to no reward other than personal satisfaction and the occasional beer and pizza. I doubt that many of the overweight and out of shape divers I see these days could even pass the required DM swims........let alone, humping gear and "herding cats" all day in an OW class.
Thank-you to the DM's and for all you do!