I had the U3, now I have the U4. Both of them work through simply using them as USB-storage.
I have read that usually libmtp should be necessary, but found this not to be the case for me.
I've also read that there are firmwares for the U3 which make it possible to have it act as a USB-storage-device. So probably mine came with that.
As for libmtp, I found that Fedora lacked the USB-IDs in the config of libmtp as well as in the config of HAL, which triggered the device not even to be recognized as a multimedia-device, but simply as a storage-device.
So, if you find yourself in the situation that it still doesn't work, even though you installed libmtp, check out the "signature-files" where the players are specified.
My U3 had a slightly different product-id than the one I found in the file.