I use a thermoworks 'Signals' thermometer... It lets me save 'sessions' (log files) of each cook. Over time I can look back and get a good feel of how long things will take (based on outside temp, meat type, etc).

It also told me what I already figured out.. The temp my smoker showed on its controller was off by about 20-30 degrees. Once I started using what my (quality) thermometer said the smoker temperature was... the quality of my cooks got WAY better.