I have done this route in both directions in april 2003 and may 2004. It's one of the best in Morocco definitely. There is nothing particularly challenging driving-wise except for some small soft dunes about 4 feet high possibly combined with muddy wadis which may increase your stress levels and throw you off course. They go on for 6km or so half way along the route, which is quite a long one for Morocco, incidentally.
Your biggest challenge will be navigation, so you would be well served by getting yourself a GPS, preferably not a handheld with a small screen, if you haven't already got one, and following the waypoints en route.
People who know this route may say that you can drive it un-aided, but I would advise any beginner, if that's what you are, to at least travel with another car, and use either a gps OR hire a guide. You will also need suitable tyres in good condition, and spare water (about 7 litres per person per day including washing/cooking). You can track down a guide or maybe another hopefeully experienced car to travel with at one of the many auberges that line the western edge of the Erg Chebbi near Merzouga.
If you want gps waypoints, they are in Chris Scott's book for this route (at least the section from Merzouga to Tagounite, which means that from Tag to M'Hamid you'll have to drive on the blacktop if you follow the book route).
You can as I'm sure you are aware go all the way to M'Hamid by piste. I'll be doing this and other routes in the area probably in march next.
Incidentally I found that daytime temperatues in apr/may were up to 39 edg C, but nights can be chilly.
Hope this helps, email me if you want,
[This message has been edited by Andrew Baker (edited 18 October 2004).]