I have one and the manual. I needed to recalibrate speedo after tires, so it was a no brainer. But it does help towing and/or mpg, timing of fan on/off, raises rpms for winching (if needed). I would buy it again if I had to do it all over again.
2010 JK. Rubicon Express 3.5" superflex, 35" BFG KM2s on Mickey Thompson Sidebiters, Poison Spyder Rocker Knockers, Poison Spyder rear Rockbrawler II bumper, Poison Spyder front Rockbrawler bumper with Warn M8000-S, AEV Snorkel, Spartan front locker. Oh, and tons of "Trail Tattoos!"