The Souk Bus:
First, it is misnomer. Souk is market, but I know almost no PCV who uses a bus to get to their market on a regular basis. Also, the buses run (sorta) on a daily set regular schedule, whereas market is weekly. It should be called a big city bus, because that is where it will take you.
The souk bus is not unlike Forest Gump's box of chocolates. It is a bus, that much can be determined before hand. And, if you take the same bus repeatedly, you can get some idea of what trip is ahead of you. But, if you are getting on a random bus for the first time, you can find yourself anywhere on the comfort spectrum.
The bus station should have some idea about the time's and destinations of the buses that travel though, but times can be "approximate". If you are visiting a rather obscure town, it is best (as always) to reference the closest major town. And arrive early, you never know when things might be shockingly working ahead of schedule.
Buses start and stop (in theory) at a bus station. Each major town has a bus station, or at the very least an area that they start and stop in. My town has a section of road that buses will pull over into, or a Auberge parking lot. This does not mean that boarding and exit is limited to these clearly defined areas. Bus's can be flagged down anywhere on their route with a raise of the hand, or they can be stopped with a loud double clap of the hands. This can be 1km from the station, or in the middle of the desert. When these bus's enter a major town their stops can become so frequent that they seem to resemble a common city bus that is slowly working itself towards some faraway goal.
You will likely end up paying around 5dh for baggage to be stored under the bus. This can be fine for a long trip, but shorter engagements should be negotiated down a bit (something I'm still working on). I have even seen things as large as a washer/dryer unit being unloaded off of souk buses. Today I watched goats get loaded underneath. I have no idea how much it costs to send a goat.
The original plan you have your own seat. This is not a guarantee, but the bus driver tries to let people know if it is standing room only. Tickets can be bought either on the bus, or at the bus station before hand. Either is fine. I have never bought a ticket then found the bus wasn't going to stop, but I suppose it could happen. This does lead to the issue of buses not stopping. For reasons only clear to the driver, there is no obligation to stop at any town. I have awoke only to find myself flying right though my destination on more than one occasion. Sometimes buses that have always stopped in a town, will decide that they are not putting the brakes on today, requiring a minor reworking of your travel itenary.
One problem that gives souk buses a bad rep with some PCVs is that they are not entirely clean. Where as a grand taxi tends to get cleared out with each trip, the bus might only get a cleaning at the end of the day. So if you are traveling in the afternoon, it is highly likely your seat has already had 2 or 3 passengers. And one or more of them might have spilled a drink or left you some candy wrappers. Additionally, the mountain passes can be stomach churning for any normal person, and downright sickening for those who travel infrequently (most the people on the bus). It is not for nothing that the ticket takers carry around empty black plastic vomit bags. Etiquette says that full ones should be thrown out the window as quickly as possible.
Another issue is that the windows and ventilation is often poor. It is a misconception with some Moroccans that drafts and cool breezes in windows can cause illness. Or let in spirits. Either way, windows are often shut and the heat inside can become very uncomfortable during the summer. This can be an issue also on the Grand Taxi, but it seems more acute to me on the buses.
The major advantages is that is can go a looong distance and you only need to pay once. A trip that might require 3 grand taxi's can be done entirely on one bus. I am not allowed to travel at night (safety first!), but a bus at night could in theory take a tourist person a long distance and they would arrive somewhat refreshed in the morning. The biggest reason to take the bus is that it is cheap. By Moroccan standards or by American standards it is a cheap ride. I can travel from my house at 7am, to Meknes at 630pm for around 100 dh (11.75US$). Thats a lotta go for little dough.