I have owned Amarra Mini for a week now. The price incidentally is now $295- plus $50 for the USB dongle - for use with multiple computers. Version 2.0 also just came out, and it supports MP3s and has FLAC conversion capabilities although I am still using Max..
I found it very difficult to audition Amarra Mini with the silences every 40 seconds or so, but with the reduced price, plus the fact that I am more or less committed to my Macbook Pro and iTunes, any enhancement is probably worth that investment. To Smeggy's points though, it is not necessarily an immediately noticeable improvement. So far, it seems to me that it tightens up the bass somewhat and overall is now more neutral sounding to me compared with Apple's Core Audio/ iTunes. Of course hearing an improvement with Amarra Mini may just be my imagination, as obviously I would like to hear an improvement after buying it.