Concept: 3 out of 5
Execution: 3 out of 5
Yeah, but: They're useful, cheap, and will actually improve your photographs. What else will do all that?
The Long Version: Grey cards and colour references aren't exciting things that photographers get together and talk about. Even dedicated gearheads can be intimidated by digital colour management, and for almost everyone the camera's idea of 'close enough' will be good enough. The more trusting photographers generally capture jpeg images, which institutionalizes the cameras' settings, and raw shooters have a 'fix it in photoshop' approach that leads to bad photographic hygiene.
But the reality is that a vast improvement in colour accuracy isn't difficult, and doesn't need to be a gateway to spyders, profiles, and multi-thousand-dollar monitors. This is easy. Sure, if it's art, go ahead and make everything green. But wouldn't it be best to know for certain what the scene actually looked like first?
I fully confess to being one of those people who shoots in raw and sorts them out later. And I'm not always the most diligent, either. But when colour accuracy matters, I start shooting frames like the one above. I can then select that photo and all of the ones taken in the same light - the ones after it, until the next grey frame - and synchronize the white balance in Lightroom without ever leaving the Library module. Six clicks and each set is done. I can colour-balance an entire shoot in less time that it has taken me to write this paragraph.
For people who shoot in jpeg capture, the sequence is even easier and more important. Point the camera at the grey card, press whatever buttons you need to to set a manual white balance, and you're done. Naturally you need to change it when the lighting changes, but that's true with anything other than the camera-knows-best Auto WB setting.
The main grey reference that I use is a Lastolite ezybalance [sic] folding / collapsible grey card. I have the small standard version, but they also make a waterproof model for dive photography, which I wanted until I saw its price. Their website is unusually useful, featuring some videos on how to use the product, and any gray card will work with the same instructions. For macro and small product photography I'll use one made from cardboard, which I've cut down to fit in the frame, and other times I'll go all-out and use a colour chart. But for general use I really appreciate the easybalance, as it's proven to be durable and the white side is also very handy for acting as a small reflector. It deploys to its full size with a quick flick, and the 12" size that I use is large enough to get an easy reading from. When I'm shooting it's natural to just tuck under my arm, or if I have two hands free I'll fold it and tuck it into my back pocket or camera bag. One of the larger sizes may be better if you need one to gauge the proper exposure, but I don't worry too much about that. After all, if I pay attention to the highlights, I can always fix it in post.
Updated to answer Jigme's question:
There are a couple of ways to shoot a reference frame. The one I use is to fill the frame (with a telephoto like the 35-100/2) by holding the card out at arms-length and trusting my E-3's auto white balance to get the temperature mostly right. (The E-3, like the E-1 and some Nikons, has a separate sensor to evaluate the ambient light.) This is the way to set a custom/manual white balance, but it's not the usual way to use a grey/white reference.
Any other time, such as with a wide-angle lens or with flash, is to include the grey card within the frame without actually taking it over. So when you have the lighting right (for studio flash) just include the grey card in one of the frames, and colour-correct them all from it in post. I typically do this in the last frame of the shoot, and after each major change in the lighting. If you're shooting one of those subjects that I avoid - things that move and talk - get one of them to hold it at some point.
But if you're shooting in mixed lighting, there's not much you can do for 'correct' white balance. All you can do is try to get the look right, which is going to be subjective and personal. A tool like this can be used to get the main subject 'right', but it's one of those times when good judgement is more important.
Carl Weese has written an excellent pair of articles on the subject over at TOP. Here are the links for Part One and Part Two.