I have been using PureMusic since version 1.6, and have the current 1.72a, and use it on 2 computers. They let you use your one license on the computers you use.
Used correctly (hog mode, memory play, pre-allocation, +/- upsampling, I find it to be a real upgrade to standard Mac audio via a DAC. Price is not too bad either. I have been too cheap to spring for Amarra, and will not usually support any software with that type of copy protection, but I read that it works well also.
PureMusic allows you continue to iTunes with all its features, remote controls, play lists, and just replaces most of Core Audio as I understand it. On my home computer, I play system audio through the speakers in the monitor, and use the USB audio driver for W4S DAC2 to be the sole output of PureMusic. This is controlled using the Midi utility.
To some degree, getting the music to "pop" with PureMusic is similar to how you had to work with Foobar 2K to get true bitperfect out, when its working right, your head starts nodding, the difference really is noticable.