do a search on the vibrant forum - someone posted the results of speed tests on the different cards by mfgr and surprisingly, sandisk's class 4 were incredibly fast, by a factor of 3 or 4X over cards that were rated much higher
the Kingston class 10's performance was a joke, iirc - performed iirc, something like a class 4 or class 5 (if a class 5 existed)
now for the down side - the sandisk class 4 cards are (or were hard to find) about a month ago - it might have changed now. But i bought two, one after the other and both showed up, and they were marked class 2. One of the outfits, Kompubuy on amazon, showed the class 4 card and the appropriate model number. what they shipped was a different model number and it was class 2. When i queried them on it, their response was "sandisk had a typo issue some time back" - there were others reporting the same issue with kompubuy
second card from a different vendor, even though they confirmed what they had were class 4 marked, shipped correct product model, but card in package was a class 2. So i called sandisk and got the run around - the tech would not answer the simple question "are you telling me that cards marked class 2 are running class 4 speeds???" - he kept obsfuscating the answer. I suspect the class 4 cards are selling like crazy and they're or were overstocked with class 2.
their class 4s btw, were testing at 15 - 18Mb write speeds where class 4 is supposed to be 4Mb
when i tested the sandisk cards marked class 2, write speed was, iirc, about 5-7 Mb, which is surprisingly fast for it's rating. But that was in line with the speed testing others had done. The sandisk cards are way under rated, speed rating wise
in actual useage, folks weren't noticing much of a difference between class 2 read speeds and the higher classes - i believe the class speed rating becomes more critical in the write mode, which, practically speaking, isn't much of a factor on a phone
search for the thread and you can read the results for yourself