If you are looking to spend some money the iGrill will measure ambient temp as well as the temp of the food. There is a separate probe you can buy for ambient ($20) but you can use one of the probes that come with the device just as well. Just stick it through a piece of potato or something and set it on the grate. The Maverick ET-732 is another similar remote unit that a lot of Brethren seem to use and it is similar in price.
The simple dial thermometer does work but it's been my experience that they get smoked up over time and you can't read them...but being cheap they are easy to replace. Sometimes simple is better. The bigger disadvantage is having to open your cooker to check the temp.
Golden Dome UDS "Dorthy
" | ~22" Char-broil "Char-lene
" | iGrill | Supersonic Blue
Where there is love there is barbecue. – Gandhi