As most photographers know already, ideally everything in your imaging system from camera capture to printer, should be calibrated and color managed. It's a nice ideal, but few of us have the time or energy to do that regularly enough for it to matter. But this checker is simple to use and I think that working it into your capture/download routine it is a good step toward completing the loop and at least getting the camera and the monitor speaking to one another in the same color language. I do use a Datacolor Spyder already, to calibrate my monitor, but I know that I don't do it often enough and also, I've never considered calibrating the camera--and it's a great idea. Hopefully this will help me mend my wicked (and lazy) ways.
This color checker works just like a standard Macbeth/X-rite color grid (though the X-rite has 24 color squares and this one from Datacolor has 48): you place the checker in your scene and in the same light as your subject and shoot a close-up frame of it, filling the frame as much as possible (and then you shoot your subject normally without the checker, of course). When you download the images, you first crop the shot of the checker (there are white dots on the device itself to use as a cropping guide) and import that image into your RAW import software (I use Photoshop, but I'm using the Adobe DNG converter to open the NEF files).
You then export the image to the SpyderCheckr software that comes with the kit and then "save" that shot as the calibration shot. Then, when you relaunch Photoshop (or Lightroom), you'll find that the calibration has been loaded as a preset in your software. The calibrations from that shot can then be applied to the photos of your subject. In essence what you're doing is color correcting and adjusting white balance by giving your editing software a true white, medium gray, color, etc. I've only tried it a few times but it does seem to work very well. I have to get used to using it more though so I can play more with trying to match colors instead of just working with the neutrals (which is primarily what I've been doing). You can, of course, also use the "info" feature in Photoshop to measure the numbers of various colors (middle gray, for example) and compare those numbers to the numbers in your images--and that should teach you a lot about what a medium gray really looks like (in a landscape shot, for example), or what a true black really looks like. (We often think we know what black is in a scene until we measure it and find out that's it's really more of a dark gray, for example.)
Anyway, it's all a learning process and I'm determined to figure out how to get the colors on my monitor to print the way that I see them--and I think that bringing a color checker into that system is a great idea. (By the way, when they say "calibrate your camera" you're not actually adjusting anything in the camera, you're just getting a true color standard so that you can compare your camera's results against known colors.)
The checker itself is beautifully made, I think. It has a nice hard shell case like a little laptop and it fits easily into the back pocket of my shooting vest so I'll carry it all the time. It also has a retractable threaded stud (took me a while to even discover that) so it can be mounted to a stand or a clamp, I gather. I haven't tried that yet.
I think the price is a bit high, but there is a lot of info on the DataColor site that you have access to with this checker and I think if you spend the money to put your color on the right path, it's a very worthwhile investment.