I have one. Installation is pretty straightforward, as it clips inline into the factory wiring harness. It does require an additional tap into an ignition-powered wire. The 12 O'Clock Labs kit comes with everything you need, including directions.
It requires one-time programming via an auto-learn function. Basically you have to run the bike through all gears at a fairly steady RPM so it knows how to map RPM to speedometer sensor pulses to display the right gear. Because it requires input from both speedo and tach it only displays the correct gear when the bike is moving. When you come to a stop it displays "--".
I find it most useful an occasional reference, usually in the middle gears. For example: When I'm trail riding and approaching a tight or steep section I want to know what gear I'm in so I can plan ahead to shift through the coming terrain. It's certainly more of a convenient indulgence rather than a necessity, but it's lightweight and I'm at the point in my life where I can enjoy a few indulgences like this
http://shop.12oclocklabs.com/index.php?route=product/product&product_id=124