I haven't had a proper trackIR, but i did do some experimenting around with FreeTrack and a home-made IR headset 'clip' and a budget web cam. It worked ok, i suspect it is not as smooth as track IR and does take more processor cycles to run etc, but it did give me a taste of what this sort of head-tracking system is like. I know it would take a while to get used to the new paradigm, learning new muscle memories etc, but i just never felt comfortable with it.
My biggest complaint was a lack of tactile reference. What I mean by that is that when you are looking backwards and up with a hat view system, you fingers are pushing the hat switch back and another finger is pushing the up modifier button etc while you are looking at essentially just blue sky. And for each change in view, there's a corresponding change in finger positions which is a learnt position which relates to the view mentally.
So when you are scanning around wildly or tracking a con through the view system you know where you are actually looking based on what you are doing with your fingers (or at least that part is a big help). Often there is no physical reference markers (espc in a P51) like canopy frames etc to know where you're looking, so i rely on what my fingers are doing to cue me as to where im am looking.
Why does it matter? Well, if i know where im looking, i can use that knowledge to make sense of how the world is moving around me in a relative sense, and i use that info to help fly my aircraft while looking, say backwards and up, or left upwards etc. Without knowing exactly where im looking, i find it hard to fly my aircraft while looking in unnatural directions.
I often just simply crashed or got into wierd stalls etc because i couldn't tell exactly what attitude my aircraft was taking. That combined with the less than brilliant performance of my crap webcam, i gave up on it and went back to hat switch. Much more comfortable.
One thing i found that was to disable (or majorly scale down) the roll axis, as i found it not very useful and only ever complicated things. It also took me ages to get a setup/scaling/profile that worked for me. This is what can make or break your experience i think.