I've had my MES 40 for one year now, and it's been wonderful. I've finally figured out what works for me regarding the vent and the external Masterbuilt "cold" smoker unit. As Bear and others have said, I close my vent for preheating and storage, and then I leave it at half-open when smoking. I tweak it a little, sometimes opening or closing it a little, depending on how the temperatures are going. But I sure love the smoker.
I'd say that my only problem is when I'm smoking a lot of meat that's cold, the smoker can't seem to recover temperature-wise. The MES might read 275, for example, but the real temperature is 225. That's almost always fine with me. And I now have a Weber grill and a Slow N Sear for those times I want a higher temperature (chicken wings!).