I have three WSM's. Two 22.5 and one 18.5. Have had them for about 18 months, many cooks. I considered getting a ATC until I cooked on them for a while.
As previously mentioned, you really don't need one. I cook anywhere between 225 and 300 and hold what ever temp I am aiming for with no problem for 12+ hours without having to reload coals. Biggest issue is when it is windy or cold and windy, wrap it up, or block the wind and you are fine.
You only fail if you do not try.