Concept: 2 out of 5
Execution: 2 out of 5
Yeah, but: It's pretty much neutral.
The Long Version: This cardboard grey card is marketed by Delta 1, a company out of Dallas, Texas. Their website is a little short on details; apparently the page for the grey card didn't make the transition over from Geocities. But the packaging comes to the rescue, saying that it has a reflectance of 18% (+/-1%) 'on which all external and internal light meters are calibrated.' There's more here by Thom, which is interesting reading if you're planning on using these for their stated purpose: getting accurate exposures.
The other way to use them is as a grey reference for white balance. The manufacturer doesn't endorse this, but my simple comparison tests show mine as being colour neutral - or at least close enough that it doesn't make a difference. It may not be as reliable as my new Passport, or even my Ezybalance, but my motto for colour management is "close enough is good enough" - it's just that "close enough" depends on the situation. This cheap cardboard will get you pretty close.
B+H sells a single one of these for about $5, and Adorama sells a pair for $12, unless you get them via Amazon for $15. While that's a bit spendy for grey cardboard, don't think about it as buying two cards, think about it as 160 square inches of eyedropperable surface. Buy one set, cut them down into little pieces, and get a lot more milage out of them. The smallest useful size will depend on what you shoot, but conversely it's about the only way to get one small enough for macro photography. My favourite size is 2"x3.5", so that they fit into my business card case. Since I have one in my camera bag(s) anyway, I'm never more than arm's reach away from a reasonably right white balance.