I got one, here are my feelings:
audio is not shinny on zero vs another highend soundcard, the mixer is a bit noisy and some numeric glitchs can ideed occur.
also the headphone out is horrible with clics and low gain. stability is half decent.
however you can get a better quality by bypassing the output convertissors
using an optical out with a better DA convertor (i used a tc with noticeable enhance)
(still very usable by default for live uses, but not top notch like a mackie or allen&heath)
Pro: Lot of midi hw, faders and cross are ultra fast dj styles, knobs and mute solo cross ab is very cool.
encoders are push buttons also (however the touchscreen, build in fx and eq are total joke imo).
so i would say the really cool thing is the modularity audio-midi, and it's an awesome cool midi controller
to have all under the hand, with faders i really find the best for my kind of fast use,
but as audio unit it's not terrible, still workable. I would say for the price you can get it now second hand it
can really worth cause they higly droped, but not when it was full price if you see what i mean
there are lots of alternatives: Allen&Heat Xzone 4D, Maudio NRV-10, Eclerc EVO_5, M_audio ProjectMix IO, yamaha, Mackie Control or onyx series, D4,Alesis Master control, euphonix, novation remote Sl, evoluction UC 33, BCF 2000, VCM-600...
my choice for cheaper would be BCF or UC33
for modular/price would be the Zero4 or 8
for DJ would be the evo5 / Mackie mixer D2 or D4
for highend live or dj would be allen&heath
for studio would be mackie control.
it depends on your uses, if more audio or more midi, booth, studio/live, budget ect.