Provided you do not have a software issue (like the EQ being on, or "sound enhancer") both which really destroy the signal, and have the output set to 16bit/44.1 (it defaults to 24 bit in 10.4 and 10.5)... you should get pretty good sound.
Personally, I find windows using foobar and ASIO or kernel streaming to have more depth and smoother... You might play with amarra or pure music, though iv'e tried neither.
As for hardware Ive tried quite a few with various dacs (benhcmark dac1, music hall dac 25.2, m-audio superdac 24/96, musiland md-10)... I avoided the usb interface on the dacs if they had one for the comparison.
m- audio audiophile firewire . Very bright. Pretty clean. Needs Power supply to sound its best.
Pop/Pulse. Uses a older usb interface... terrible quality. Nothing good to say.
Lite audio usb... odd cable setup... but sounds pretty good. not bad not great.
m2tech. (BNC), better than the trends, and no usb cable to buy. does 24/192. Clear winner (I set it to 16/44 to be fair)
trends ud 10.1. excellent depth and stage.... not quite as smooth as the m2tech but not grainy either. External power supply makes it brighter sounding, not necessarily better.
overall the only ones I was "happy" listening to were the trends ud10.1 and m2tech. I found the usb on the music hall and trends to be somewhat lacking compared to the trends and m2tech.