Bought 2 of these after the 5.3L in my '08 Tahoe dropped 2 cylinders at 147K miles. Stuck them in my Tahoe (after a new engine) and '14 Silverado to make sure I don't have problems in the future. Maybe slight decrease in fuel economy, but doesn't appear to be much. Provides some peace of mind which makes it worth it for me.
I am pleased with the way this device works. I have had no problems with it as some other people have. I can feel the difference in performance. I check fuel mileage towing a small trailer with 2 rolls of hay and 20 bails of hay. I averaged 13 mpg. With the ac running all the time. I have let the Tahoe sit for a couple of days with no problem starting. I will unplug if not running it for a week. I've only had it for a couple of weeks. So far so good. Worth the money in my opinion.