Simply put, a temperature controller will make smoking on the WSM pretty close to "set it and forget it." I use one on my WSM, and I am happy with it. Other folks like to tinker with their cookers.
For thermometers, there are instant read thermometers, like a Thermopen, that you use to check internal temperatures intermittently. There are also probe thermometers meant to stay in the meat throughout the cook. I stick a wired probe thermometer into my meat, and leave it there, to monitor my cooking progress. When the IT gets pretty close to when it could be done, I start probing the meat for feel, not temperature.
With either kind of thermometer, don't use internal temperature to decide when the meat is done. You need to go by feel. Your probe should slide into the meet "like butter."