I just picked up the Apogee Duet 2 mainly for recording but have been using it heavily for playback and love the quality. It makes such a difference in sound quality that I find myself carrying the thing back and forth between my office and home each day so I don't have to ever plug directly into the mac sound card...
wgscott is right that just about any external DAC is an improvement over the internal DAC in a MacBook, which is surprisingly poor. If you're using your computer to playback audio, a decent DAC will make a significant and noticeable difference.
I've also liked how smoothly the duet integrates with OS X. Other setups had me using the MBP's optical output to power a Musical Fidelity V-DAC but the Duet sounds better, plays well with OS X and has a ton of other great features.