If the thermapen was the only thing thermoworks sold, I'd go with them. But they sell other equipment that's better suited to homebrewing!
Rather than a thermapen, get
this small handheld meter, combined with the
"THS-113-372-T" PTFE/FEP TIP PROBE ("Use ... anywhere a waterproof thermocouple is useful.).
Combined, they're about the same price as a thermapen, but the sensor is submersible and you can leave it in your mashtun, hlt, etc for as long as you'd like. Get more than one sensor, and you can move the meter between them without disturbing the process. And for another $30, it makes a great meat thermometer which will make next years' turkey a lot better.