My husband and I just returned from a 2-week trip to Morocco. We rented a car on two occasions: for a semi-loop itinerary from Fez to Marrakech and later, from Marrakech to El Jadida (north of Essaouira as you may already know) and then up to Casablanca. No issues whatsoever, so long as you're a safe driver and that you follow the speed limit. (Driving into Casablanca was probably the most challenging part for us because we were staying on the coast and there seemed to be a lot of activity there, but apart from that, it was smooth sailing!) The roads that we were on are well-maintained, and you'll be able to take a toll road for part of your drive up to Casa. We really enjoyed the driving aspect of the trip.
One tip that I found really helpful from this forum was the suggestion to download the app 'maps.me', ensuring that you download the Morocco maps you bed before you go. It easily showed us which way to go - we even used it while hiking in the Todra Gorge! Highly recommend.
Safe travels, have an excellent time.Edited: 19 October 2017, 14:56