Seems to get debated a lot. I think a good one improves the sound - seems to make it a bit fuller and smoother, more CD like I suppose. I use one (an Arcam irDAC, which is a fairly expensive one) and I'm glad that I do. I put my CD player (a reasonably good Denon one) through it and can easily switch back and forth between the CD player on its own and with the DAC. It's a slight but definite step up in sound quality with the DAC. For MP3s off my computer it makes a definite improvement and I also use it for my TV.