I use my RR2150 almost exclusively with the USB input (my G5 iMac is the principle source in the RR2150 system) and have had no problems whatsoever. Worked automatically, without any configuration (Mac sees it as an external sound card).

Unlike the previous poster, I think quite highly of the sound produced by the USB input. To get a handle on it's quality, I used the RR2150 for a short while in my "main rig", where the USB input was compared to my Mark Levinson 390S, both fed from iTunes running on a Powerbook. The RR2150 acquitted itself quite well.

No, it didn't have the bass slam of the Levinson, and was less musical overall, but plenty enjoyable. And, as I said above, I use the USB input every day, although not in a "critical listening" application. It beats the sound from the iMac's own soundcard.