I don't have the headtracker yet (apparently get it next week or so). Tried it with the camera. Works ok. I'd give it a C+ for simulation believability and probably an A- for quality.
 
The sound quality is pretty good overall. The room ambiance sounds fairly realistic, not really like a synthetic reverb (which it is, of course). Even cranked all the way up, it doesn't get that nasty synthetic sound many do. No obvious artifacts or messed up sounds like you get with some of the crossfeeds. It sounds good with basically any settings.
 
It is somewhat believable in terms of sound. It does help take the sound out of your head, but it doesn't make it sound just like speakers in your room. Maybe with different/better headphones it would be better (I use Denon AH-D2000s) and I'll also have to recheck my head measurements. It works, but it isn't the "wow that sounds totally real" effect you can get with something like the SVS Realiser. You still get things that sound more like it is in your head. In particular cymbals seemed to stick back in the ear cups more. Also low bass, but that was more that you feel it from the earphones in a very different way than speakers.
 
I imagine it is more useful for surround than for stereo. When I panned the speakers behind me, it gave a pretty reasonable "sound coming from the back" effect. So I imagine you could mix in surround using it and have some decent results. You'd want to check them on surround speakers, but it would work. Also should work pretty well for playing games with surround sound at night, which is what I really got it for. You can't use surround in Sonar near as I can tell, since Sonar doesn't have support for surround VST plugins.
 
For stereo I'm not as sold, I don't know that the effect is dramatic enough to make it worth just mixing over headphones. You'd have to see. I'll play with that more when the tracker comes in.
 
The headtracking does help with the realism, but it doesn't work all that well, at least with my camera. The headtracker is supposed to work a fair bit better. It tracks your movement over a full sphere, and has faster updates. Doesn't track your XYZ position (you can use both together if you want that) but that part isn't as important for realism cues.
 
So I dunno, worth trying the demo at least. If the headtracker works well, probably worth the current asking price for both ($99). Don't get me wrong, my surround sound system is staying in place, but this looks to be a reasonable thing for late night messing around and on the road.
 
Also for people who do real studio work, not just playing as I do, it could be worth it for musicians. With the trackers it can supposedly do like 6 people at the same time, giving each their own tracked version so could be useful if musicians want something to try and get the music out of their head a bit and still have the isolation of headphones.

Thanks for posting - I asked about this in a separate thread yesterday. I am only interested in stereo mixing, and maybe there are better solutions, but it will be interesting to get an update once you have the headtracker.

I'm not interested in tracking either.  I've tried NX out a bit for simple stereo use and it does a pretty good job making the stereo image sound more like speakers.  It obviously does nothing for the response/quality of the phones.  I could see NX sans tracker helping quite a bit on the tracking side.
 
Time will tell 

Tracking is worth looking at just because it enhances the realism of the stereo image. Part of the problem with doing HRTF is that our brains rely on cues from how audio changes as we move our heads and we don't get that with static headphone virtualization.

Just remember to remove it from the FX chain before rendering.
 
Don't ask. 

So I finally got my headtracker (UPS issues :P) and man... WORTH. It upgrades the believability to probably a B+ as well. It is seriously good. When I get it all set up, it is good enough that as I rotate around in my chair the sound seems to stay on my speakers. In particular things in the mid range that are hard panned right or left sound so much like they are coming from my speakers I took off my phones to check and make sure I didn't have my speakers active.
 
It really does a very convincing job fooling my brain in to thinking that the sound is staying fixed in space and isn't coming from right beside my head. It isn't perfect still, high frequency decorrelated sounds like cymbals still seem to localize to the headphones too much but in general it does a pretty good job of moving thing out of the head in a convincing way, even things line snares and toms. Seems to work a little better on mixes that aren't as dense and bass heavy but works on everything I've tried so far.
 
The setup I find makes it work well is:
 
--Measure your ears as they instruct with a flexible tape measure, don't use the built in head model.
--Match your speaker position in the plugin to the real position. In my case that's 90 degrees, corresponding to speakers each 45 degrees off center. It defaults to 60 degrees, the THX standard.
--Get the tracker on your headphones as centered as you can and get it enabled.
--Make sure your phones are on your ears properly, lined up right, etc.
--Set your head facing front and leveled and hit the "sweet spot" button.
--Turn up the room ambiance to around 50.
 
The room ambiance is a big part of it sounding realistic. Down to 0 and things stick in the headphones a whole lot and just kinda slide around your head. At 100 it does the best job of pushing things out of your head in general. However the tradeoff is more room effects including reverb/echo type sounds and muddier bass. At low settings, 20-30, the bass is more like what I get in my actual room (which is a pretty garbage room, but with some treatment, massive speakers + sub, and high end Dirac electronic correction), but the realism usually isn't as good. I mess around with it, but in general I like 40-50ish the best so far. There I am getting a pretty satisfying effect of not being in the head, and good sound overall.
 
I am still not going to dump my surround setup... but if I was forced to now I wouldn't be nearly as mad. This is an acceptable replacement for speakers in general. Not perfect, but pretty good.
 
That said, no matter how much I've played with it so far, I can't get the stereo image both as focused and as real as my speakers. If I crank the ambiance way down, and particularly if I turn tracking off I can get a super focused image as headphones can do, but I lose the realism. When it is pretty realistic it doesn't produce the dead solid, detailed, smooth image my speakers do. It gets imaging more like they do if I disengage Dirac, maybe better, but it doesn't have the totally solid sound stage they can produce with it engaged.
 
I also can't test surround with it yet as Sonar doesn't support surround VSTs, and the Nx application for general Windows sound is out yet. When it comes out, I'll test it with movies, music and games and let people know how it does for full surround. My speakers are also full 7.1, so I can get a pretty good comparison.
 
Either way, I'd get this unless you never use headphones. If you find at any time you like phones, it is worth it. IMO. Right now they have it on sale for $100 for the plugin and the tracker and I think it is worth that.