What kind of USB device? If it's a USB flash drive, 700MB is ok (about 6MB/s) but not particularly fast (I get almost twice that to my SanDisk Cruzer).
If it's an external hard drive, 6MB/s is slow. I get 16-20MB/s to my NewerTech MiniStack v3 with USB, about 36MB/s with FireWire 400.
Remember though, the USB ports on the MBs aren't equal. The one towards the back shares the USB with internal devices. Use the forward one for external drives etc.