I have always wanted one, until I got a BPS kit. I have found that my drum keeps constant temps for over 15 hours, so I haven't actually needed any sort of pit temp controller. My BPS drum is practically set it and forget it.
I use a guru which Big Poppa Smokers sells a adapter for. Here is the but part as jetmek pointed out once the vents are set right the drum runs fairly steady. So how we use the Guru: assuming cooking at 250. I will get the drum to run at or a little below that temp and only have the guru keeping the last 10 degrees or so in check. This keeps the air movement down in the drum so the good smoke stays in the drum longer. I have had cooks where the fan on the guru doesn't run for hours. it is just there as a insurance policy.