I transferred about 14GB of mp3 files from my wife's computer to my PB which was in Target-Disk mode. The whole transer took about 1/2 hour roughly. The only thing I had a problem with was that I had disabled the boot keys and holding down the 'T' wasn't having an effect. Once I fixed the security, it worked like a charm. Make sure that you 'Eject' your Target machine before you disconnect the Firewire cable.
--Dr_Doom