I'm not a BBQ or WSM expert, but I do know a thing or two about controls and would like to clear up some misconceptions I've seen in this thread.
First, the difference between on/off and PID. In the on/off mode the fan will blow continuously until it reaches the set point (say 250 for our example) and will stay off until the temp drops to a lower point (say 245) that will energize the fan until the set point is met again. This control scheme will much more likely result in temperature over runs. The fan may stop at 250, but the fire doesn't. There will be a period of time after the fan stops that the temp in the cooker is still rising. The particulars of your setup will determine how bad this over run will be. A PID controller takes this into account and will start cutting the fan output as it gets closer to the set point. To the best of my knowledge, the fans that comes with most of these BBQ systems do not have a variable capacity. They are either on or off. The way the PID modulates the fan output is how many seconds per minute that the fan is blowing. If you are far from the setpoint (say 200) then the fan will probably blow nearly continuously, if you are close to the setpoint (around 240) the fan will blow intermittently. This allows the PID control scheme to approach the setpoint more slowly where the on/off would blow continuously until setpoint.
For the PID controller to operate properly, there are a parameters that are already programmed into the unit when you get it. They are factory tuned to operate most smokers well, however they give you the option to let the controller run the the "autotune" routine which will overwrite the PID parameters with new values that are specific to your setup. Since the same model number controller (1614) is sold for different types of smokers, I can only assume that the parameter presets are different based on smoker type.
I hope this info is useful to someone.
Now, for possibly useless advice from someone who has never used an ATC. BE CONSISTENT!!!! There has been a ton of advice about water pans and dome vent position, but no one has said to be consistent. The controller should be able to compensate for variations in the system up to a point. To get the best results out of a controller like this is to set up the water pan, vent and build the fire in a way that is repeatable. I assume that the hardest part of this is to build a fire in a way that burns evenly and doesn't cause flare ups and temp over runs. Also, when running an ATC, DON'T MONKEY AROUND WITH ANYTHING. The ATC will never work if you are messing with the vents or opening lid regularly.
Please take the last paragraph with a grain of salt since I've never run one of these before.
