First off, it looks like they require premium fuel only (newer models for Chevy support the dual fuel setup -- 87 or 93 octane). So gas mileage or not you'll run the more expensive premium blends...
That being said there are always differening opinions on how these things work. Personally if I get one of these for my full size it's just to get rid of the speed limiter on my truck (hate it when the thing kicks off at 97 mph, but that's probably a good thing for me).
And I hear most people say "don't waste your time" and put in the other mods, like opening up your intake and exhaust to let it breathe better, and the best is to get the proper gearing for your tire size (i.e., 3.42 gears SUCK with 31" tires -- get 4.10's).
Anyway, if you haven't gone there already, google is your friend --