I came from using Innuos' previous version of the Zen Mini. The reasons I originally went for the Zen Mini were:
get rid of all CD cases to gain space
flexibility of editing via my iPad
direct connection via USB to the DAC in my pre
Innuos' excellent support
I went for this after adding the Classe Sigma SSP AV pre to my system, which incorporates an excellent DAC, and with the ability to connect the Zen directly to the DAC in my pre, I didn't need to stream anything over my network. Having encountered differences in CD transports, this approach completely removes that influence.
Then the Zenith appears, with better power supplies, solid state storage, and all the other stuff Gazzip has already listed. The Zen Mini was quiet, but the Zenith is totally silent, which is good as the unit sits right behind me. I knew this would get me as close to a perfect digital source as possible for a reasonable amount of money. The old Zen Mini was good, and I was happy with it, but moving from that to the Zenith has been one of the best moves I've made. The difference between the Zenith and the current Zen can be quite clearly heard, which was the subject of a little event we held a few months back (playing through a Classe CP800/CA2300 and KEF Blade 2s). There is no harshness or brightness to the signal, just totally clean effortless music. The combination of the lack of noise and distortion of both the Zenith and the SSP makes for a great combination with inky black silences between music, which for me, just adds to the naturalness and transparency of the whole system. I've heard a few high end CD players in store, but I can't imagine them bringing anything else to my system over this. I've lost count of the amount of times I've heard new detail in music, even tracks I've known for decades.
I've always still felt that source is still important, even digital ones, and my experience with the Zenith just hammers that home for me. One of our customers is trying one out over the weekend - I'll report back on his decision.