Depends how accurate you want to be. I was dead on to my GPS with 19/41. I am only slightly off with 18/40. When I need to be accurate I just use my GPS, seemed like a lot to go through with the Yellow box. Nice once you get them set I guess.
The Yellow Box is made by Black Robotics it's a pretty small unit (2" x 1"). It corrects your speedometer and odometer errors. I'm not that concerned about the speedo error, but I would like to correct the odometer error. No reason to register more miles on your bike than you have to.
There is a new Yellow Box v3 that has a 3-wire output and plugs in between your speed sensor and the speedo. The online instructions are not currently available. You can correct your gage error from -28% to +28%. The cost is about $100, pretty steep for a little box, but I have not heard any complaints about the product.
I currently have a 42 rear sprocket, when I change the front to a 16 I will order the Yellow Box.
I am at 16-42 and was off 19% with a GPS. Stock gearing I was off 7%. Easy to install and easy to use. I think it is well worth $100 just to keep extra miles off the bike which will increase the value of your bike in the long run!