Thermopen is great if you open the door and stick it in the meat. If you want a good/inexpensive/reliable thermometer tha you can stick in the meat and have the digital readout outside the smoker.. go to Target and pick up a Taylor digital cable thermometer. They cost about $15. They are very reliable. If you want to have one that is remote.. ie. the sending unit is as the Taylor.. a cable/probe stuck in the meat with a transmitter outside the smoker.. and the receiver in your house to alert you about the "done" time.. go with a Maverick.. they are more expensive. If you have to have something close to you.. that's the way to go.. but if you don't mind going outside to find out what the internal meat temp is.. go with the Taylor.. as I say.. cheap, accurate, reliable... and you can set it with a beeper on or off at your option.. it will beep when temp is reached.. If you have $$$, get an Amerique. You can set it and forget it. Set the "finish" temp and once reached.. it will go to a hold cycle.. about 140º and you can not worry about missing the temp and overcooking.
That's my story.. and I'm sticking to it.