All happy Kindle Fire customers, to paraphrase Tolstoy, are the same. Unhappy customers, however, are unhappy in their own way.

To get a feel for what makes them unhappy — and how unhappy they are — we spent some time Saturday morning reading the Kindle Fire feedback on Amazon.com.

There were 3,678 write-ups in all, nearly half of them (47%) glowing five-star reviews that basically said the same thing (Typical headline: “Outstanding value at $199”).

What interested us, however, were the 491 (13.3%) one-star reviews. They are relevant because the number of Kindle Fires being returned to the store is likely to be an undisclosed material factor in Amazon’s results this quarter.

Unlike Apple (AAPL), which reports unit sales to the SEC every three months, Amazon (AMZN) has never told investors how many Kindles it has sold. The best we have are guesses, like the estimate released Friday by IHS iSuppli that Amazon will ship 3.9 Kindle Fires this quarter compared with 18.6 million Apple iPads.

The satisfaction ratings for iPads, however, are off the charts. The Kindle Fire? Not so much.

Here are headlines of the top 10 one-star Kindle Fire reviews rated “most helpful.”

Total disappointment 773 of 967 people found the review helpful
Does not open previously purchased Kindle Books 489 of 614 found helpful
FRAUD on the Fire!! Deregister yours before it ships! 952 of 1,200 found helpful
Content in the Cloud. Forget it. 417 of 524 found helpful
I really wanted to like it – but it is not for me 350 of 446 found helpful
Very Dissapointed with Kindle Fire and Customer Service 190 of 241 found helpful
Using the Fire is frustrating and unpleasant experience 48 of 59 found helpful
Very Disappointed 259 of 332 found helpful
Open letter to Jeff Bezos 41 of 50 found helpful
Returned the Kindle Fire after only one day 166 of 215 found helpful

Eight of these 10 customers stated explicitly that they were returning their Kindle Fires, and a ninth sounded like he already had.

Using 90% of the 13.3% one-star reviews as rough measure, this suggests that nearly half a million of the estimated 3.9 million Kindles to be shipped this quarter will be returned.

How do the iPad reviews compare? Amazon doesn’t promote the iPad like it does the Kindle, and it only sells it through third party vendors. But of the 475 iPad 2 reviews posted on Amazon.com, more than 60% gave it five stars and only 7.2% gave it one star. A couple of disappointed customers had to return defective devices, but only one asked for his money back. He was upset because he couldn’t charge the device using USB port on his (old) Windows laptop.