I've got one on my car atm. For the $40 I paid, I figured I'd give it a go. Installed it about 3 months ago and although it may be coincidence, an issue with "hunting idle" stopped.
Other than that, I've seen no benefit to having one of these. I got the same fuel consumption etc etc. Now that I'm promted, I'll remove mine soon to see if the hunting idle problem comes back and I will let you know what happens.