Sorry if I dig up an old thread, but I wanted to comment the info given here... The class of these flash cards is the minimum constant write speed for sequential writes (big files). However, if you read speed tests of class 10 cards, you'll see that the cards are the speediest for big files, but really slower for little files. So if you need to write many tiny files you'll be well advised to use standard (class4 generally now) microSDHC cards.
It would be really interesting if someone who has a huge collection of EPUB files could do the comparison between writing 10gb EPUB files on a class4 card, and doing on a class10 card. If you write 60MB pdf files, class10 is probably an easy winner. But for small books with no image embedded, I'm not so sure
a lexar 32gb class10 speed test from external source (bad results even for bigger files in this case : for 50mb files it performs well under 10mb/s)
a standard Sandisk class2 card, for the same 50mb test
Note
for 512kb files, this sandisk class2 is almost 3 times faster than the class10 above (3.5mb/s vs 1.3mb/s)
edit : cf
http://www.xda-developers.com/androi...is-speed-test/
Class 2 and 4 cards have better random access speed and random read/write than class 6 or 10 cards in general. To get higher sequential write speeds, manufacturers sacrifice the random access speeds, speed charts, and more.