Early days as i've only recently got the card a day ago, but i've managed to copy over 30gb of music and everything works well.
In regards to the transfer speeds of this card, it really is a class 4, but the only way to achieve these speeds is not to use your phone but a memory card reader instead.
While transferring to my phone, i was getting 2mb transfer speeds (class 2), but in my memory card reader i got the full 4mb (class 4).
I suspect the reason why some people have reported getting 2mb speeds is because they were transferring over using their phone, this is a bad idea, not only because of the slow speeds but also because transferring huge amounts of data to a phone kills the battery possibly corrupting the data being transferred over when the phone dies mid transfer.