Functionally, the M51 is a home-run for me. I have it set up as the control center for the TV rig where it works quite well. Two HDMI inputs to handle the DirectTV box and DVD/B-R player, plus spdif for the Squeezebox (eventually will probably replace the squeezebox with a PC connected via USB). The fact that it replaces a pre-amp/pre-pro as long as you only need stereo is a big win IMHO.
@dwk, you have hit the nail on the head - not sure how many others want a DAC to do so much 'double duty', but for me something with that level of functionality ticks a lot of boxes. I am considering the Octave for desktop use, but the M51 definitely fits into a broader niche. Also looking at Marantz NA7004 as a 'swiss army knife' - it never ends.