I can't imagine ever going back to tuning without one...... The "look" of the plugs only tells you so much.......just like that number the O2 spits out only tells you so much....It shows lean and rich stumbles and you INSTANTLY know it's a lean OR rich stumble, instead of guessing and spending DAYS trying to dial it in....I just thought I had some of my cars dialed in, until I hooked my LM1 up!! lol

You can turn just ONE of the six idle mixuture screws on a Six Pack and SEE it on the O2 that's in just one pipe.........There's NO WAY reading plugs will tell you that.....

You can also see when your PV opens up........you can watch the motor get leaner as you tip in, then watch it get instantly rich......Plugs won't tell you that and the ass-dyno isn't that accurate........

If it's a race only car that has to only idle and go WOT, you don't really need it (but it can help)........but for anything street driven, IMHO, it's a must-have......