My partner and I booked a private tour to Ouarzazate, through the M'hidi Travel shop in the souks. We booked two days in advance, and found talking with the agency owner face-to-face very helpful.
The 4x4 was comfortable and a smooth ride. The tour owner met us in front of the Cafe de France (4x4s are not allowed in the Medina, where we were staying) so he could introduce us to our driver. Lots of tourists had booked tours and were wandering around looking for drivers they had never met, so this was a reassuring and nice touch. We only paid for our tour (we had put down an advance of 30%) once we met the driver.
Our trip took us through a breathtaking, if sometimes hairy, mountain pass in the High Atlas. We stopped at two ksars (fortified villages on the Salt Road): Kasbah Talouet, up in the mountains, the largest in South Morocco after Marrakesh; and the windswept Berber village of Aït Ben Haddou in the desert, which has been used as a film location in countless blockbusters, including Game of Thrones, Indiana Jones, and Gladiator.
If you leave enough time, your guide may push on into Ouarzazate itself to visit the Film Studios. Wandering through the maze of old film sets (Jewel of the Nile, Cleopatra, Game of Thrones, Kingdom of Heaven, etc.) is slightly surreal.
We spent an amazing, if tiring day, leaving at 7:30 am and returning just past 7 pm. Our guide was fun and friendly, and really went out of his way for us.
His English was poor, but as we speak French, we were matched with a French-speaking guide.
We paid a fixed price of 1600DH for two people, much less than we were quoted elsewhere online (the shorter excursion to Imlil was 1500 DH). A delicious three-course lunch was included.
I would strongly recommend taking enough cash to cover getting a guide for both ksars (20-40DH) and the film studios (50DH pp). There are also stops at a coffee shop (10DH/coffee), an Argan-oil women's coop, and several scenic points.
I also recommend going into the Aït Ben Haddou Berber carpet women's coop.: they sell beautiful handwoven cactus-silk carpets which we did not see anywhere in Marrakesh (starting price was 800DH, but we got ours for less). The man who ran the shop was friendly and quite knowledgeable about Berber culture. You will have to bargain the traditional way (lots of chat and tea).
Also, wear lots of layers: it's hot in the desert, and freezing up in the High Atlas, at least at this time of year.
Cannot recommend using M'hidi Travel enough.
All in all, I highly recommend M'hidi travel.