If you are travelling in winter then be prepared for some very cold weather. The hotel in the gorge and the gite in the mountains do not have any heating so make sure you have enough warm clothes, thermals and a decent sleeping bag. Hot water for showers was available though thankfully! Desert was also v cold at night so be prepared.
The gorge and Atlas mountain hikes do require some level of fitness. Although not technically difficult, you are hiking for 4-5 hours with a lot of uphill. We did hike in rain and a blizzard in the gorge and through snow in the mountains so decent waterproofs recommended!
Shopping seemed to be better and cheaper in Essouira than in Marrakesh. Prices were cheaper and they were more willing to bargain.
Although there are a couple of long travel days, you do make frequent stops. And with so much to see out the windows of the bus the time did seem to fly by very quickly.
Try to pack light for two reasons - there isnt a whole lot of room for luggage on the mini bus and you will end up shopping and buying stuff there.
Oh and dont forget earplugs - the first call to prayer at 530am can be very very loud!!!!