Very interesting blog post: http://freakonomics.blogs.nytimes.com/2008/09/29/do-good-grades-predict-success/
i would agree with many of the comments on this post that grades measure very little...as some of the most intelligent and successful (in my mind) people i know did horrid in school. my husband and father, for starters. as i recall, my bro didn't do all that great either, at least up through high school. and i certainly was no A student.
i always kind of figured that if i ever did send my kids to school, that i really wouldn't put a lot of stock in their grades, because in many ways they seem utterly irrelevant. but then, i've not sent my kids to school so i don't really know how i'd feel if i did.
i find it interesting, tho, that our society always seems to put SO much emphasis on getting good grades. sure it means they are hard workers and they are able to spit back out the stuff that was thrown at them, and do the work for the papers and such that is necessary...and that's certainly valuable in some ways. but really, how much of that will they retain? some will, sure...but many of us that did ok, say, in college (i did better in college than high school, as i recall) couldn't tell you a thing about what we learned to earn those grades. i took entire classes that the entire semester or year's worth of info is completely lost to me. i realize not everyone has this happen though.
my point is that in my mind, the importance of education is in learning to love learning. and as such, your entire life is your "education." if you come away from school, whatever form it may take (be it public, private, homeschool school-at-home or unschooler), with that thirst for learning, and have the tools to enable yourself to regularly get that thirst quenched...(such as knowing how to read, which i feel is THE single most important thing anyone learns ever) then you've gotten a "good education".
and if you love learning, in my mind, you will tend to retain much of the info that you have obtained. probably not all, but a lot. i can tell you that my education was pretty mediocre because i didn't learn to love learning til well into my adulthood. and i didn't retain all that much info from what i "learned" in school.
now, i realize many people feel differently than me on this subject. and that's ok. everyone is entitled to their own thinking on any subject. this is just mine. :)