Overall it looks pretty good.
The driver works: I tested on Debian Lenny with a djconsole RMX.
Installation of the packages from Hercules is ok, as well as the dkms package from Dell (Debian is on its way to include it, see: http://pkg-dkms.alioth.debian.org/
After installation, the midi functionality is definitely there: the djconsole controller part is now recognized by other applications,and there's a nice brushed aluminium control panel. I didn't experience any instability so far.
Now regarding Mixxx:
After compiling the Mixxx 1.6.1+Herc RC1 build (the Ubuntu packages doesn't work on Debian testing/unstable which has an older version of libasound2: 1.0.16-2), the herc is correctly detected, controls work smoothly, actually smoother than they ever did with libdjconsole. The play buttons even blink nicely near the end of a song - sweet! The only problem: the default mapping for the RMX is not ok regarding the play/stopbuttons, and the scratch button doesn't work - not a big deal.
However, the behavior of the application itself feels kind of strange: After using 1.6.1+libdjconsole for a few months, the touch of the controls feels a bit different (ok this is normal, just a matter of getting used to it), but I can now hear some audio glitches at a latency setting which was usually fine. I also have the impression that the waveform display is not moving smoothly but in a somehow saccaded manner, and the CPU usage peaks for a longer time when loading a song, in short: i suspect some performance issue, but I cannot really tell if it comes from the driver - need more testing.