I'm a little disillusioned with Mixbus. From their marketing blurb you'd be forgiven for thinking that Mixbus has its own sound just from running audio through it, or at least summing your tracks through its dedicated mixbuses. You will find claims everywhere that this summing is done with "analog" style magic.
But as Jeff Evans pointed out recently, tracks summed and exported from a "transparent" DAW will null perfectly with tracks summed and exported from Mixbus provided you don't engage any of its effects. So it's not really doing anything to the sound on its own. The Harrison sound comes instead from the Harrison effects - the inbuilt EQ and compression, the tape drive, and the various add-on effects you can purchase from Harrison. I read somewhere that the tape drive isn't even that sophisticated - it just adds some third order harmonics and engages a slight rolloff.
I've heard all sorts of claims that just running a mix clean through Mixbus will give it "more depth" and a "broader stereo spread" etc. People swear they can hear it. But null tests show otherwise. Harrison mentions its "great sounding" bus summing on its site, but does it actually do anything? I read someone on the Harrison forums saying that this summing magic might be so subtle that it "still passes a null test." But come on, would it? If anything were happening at all different, they would not null.
I've followed threads on the Harrison forum where someone from Harrison has chimed in, and I'm always left feeling like I still haven't learned anything solid about the Mixbus "secret sauce" and what it does. They're vague and ambiguous and don't seem willing to put their cards on the table and say "this is what Mixbus does and doesn't do."
My original intent when I bought Mixbus was to do all of my mixing in Sonar and just import the stems into Mixbus for bussing and summing. But if there is no magic summing per se, that all seems pointless. I still use it, because I like the idea of switching to another DAW in the last stage of the mix. It takes away all the distractions of your original project and gives you a fresh start. Plus I like to group my tracks into busses and apply 0.5 - 1dB of compression for a little extra glue before they hit the master compressor, and sometimes I will do a little light EQ on a bus as well. So when I use the Mixbus effects for this, I'm getting some of the Harrison magic, and it does sound good. I just think that if people are buying Mixbus just for its purported summing magic, they're quite possibly being conned.