The Mackie Onyx is a compact 16 into 2 mixing console, very suitable for recording straight into stereo (as any sound engineer worth his or her salt should be able to do). There have been many consoles of a similar type, but one area where they have generally failed is in multitrack recording.
OK, 16/2 consoles are not multitrack recording mixing consoles and are never meant to be. But somehow it's a shame to have sixteen mic preamps (or a combination of mic/line inputs) sitting there in a highly affordable package and not be able to use them as a multitrack front end if you want.
But in the Mackie Onyx you can! Take a look around the back and you will see two D-sub (D subminiature) connectors. One provides outputs from mic preamplifiers 1 through 8, the other for line inputs 9 through 16. Granted, the D-sub connector is not the most convenient in the world, but it is often used successfully for audio, and you can buy ready-made cables in a variety of configurations.
So you can connect the Mackie Onyx to a multitrack recorder and capture your event directly to Pro Tools, Logic, Tascam MX2424 or whatever takes your fancy. The levels of the output signals are only affected by the gain control so your recording will be pristine clean, and you can feed a PA system or make a stereo recording in parallel. Any EQ settings or fader moves you make have no effect on the multitrack outputs.
If you don't want to be bothered with D-sub connectors, or indeed analog output signals at all, then there is a Firewire output option which will make connection a lot quicker. Unfortunately, there isn't a 16-channel Firewire return, but you can always mix through the Onyx by connecting to the analog inputs. I suspect though that many users will employ the Mackie Onyx as a convenient front end, and mix within software.