I ordered my C4s in early Dec. Frogbeats received my impressions on 12/19. I received my C4s on 3/14.
I'm doing a lot of comparisons with my AS-2s.
Initial impression is that C4 is much brighter, AS-2 has much more bass/lower mid presence. On my iPhone/Vamp rig I use 'Bass Booster' eq w/ C4s, 'Treble Booster' eq w/ AS-2s. They end up sounding very similar then. AS-2 still a little warmer, C4 a bit more open and 3 dimensional staging, also a bit more upper-mid focus. But the differences are surprisingly subtle.
I use iTunes eq on my laptop/Ultra-Fi/Shadow rig -- boost bass 32, 64 & 128 Hz bands in descending increments from +8db.
Without eq, treble/upper-mid energy overwhelms bass energy on lots of songs (eg last 2 minutes of 'Blessed' by Lucinda Williams, or The Who, 'Magic Bus').
C4 bottom end responds well & 'cleanly' to eq.