While, for casual listeners, the Mac's DAC (digital-to-analogue converter) delivers decent enough quality via the headphone output, it's a low cost solution, and one that's easily improved upon in terms of noise, jitter and overall fidelity.
Arcam's rPac is a small, solidly built aluminium box designed to do just one thing: deliver pristine stereo audio output from your USB-connected Mac.
At its core is the high-end Texas Instruments Burr-Brown PCM5102 DAC, and it employs asynchronous USB (running its own master clock, rather than passively waiting for the Mac to deliver data) to do away with jitter. No drivers are required - just plug it in and go - and the outputs comprise a headphone minijack and a pair of RCAs for connection to an amp/speakers.
The rPac is a massive step up from the Mac's built-in chipset. The detail and dynamics in acoustic material come through with accuracy and depth, while electric, electronic and bass-heavy styles gain coherence, weight and muscle.
Of course, the more compressed your source audio, the less impressive the results - in fact, for low bit-rate MP3s, the ever-truthful rPac actually highlights top-end compression artefacts that the Mac chipset might smuggle through. However, with high bit-rate, losslessly compressed and uncompressed files, the results are stunning.
If you're serious about audio fidelity, the Arcam rPac is something you need to hear. Although, if you've never been particularly aware of anything being amiss with your existing output, it would be an unnecessary and moderately expensive luxury.