I got the Privia 350 in mid-October. Have had plenty of time to explore it since then w/PT. Initially in the store, the sales person and I found that we could sometimes hear the difference very easily and sometimes not. The owner of the store who really would have preferred to sell me a high-priced Roland keyboard was very adamant he couldn't hear any difference and that no one else could possibly hear a difference either! ...
What I've found in the months since then is the High-def MIDI definitely adds expressiveness. It's most noticeable for me with very soft passages, very fast single note lines, and certain kinds of chordal passages. The greatest effect I find is the keyboard feels (physically) more nuanced. Physically in the sense of how the feel of the keys connects to the sound you hear.
I've set up a test patch in Max/MSP that plays fast random notes) to compare the high-def side by side to normal def. The difference is really obvious. Fast single note lines sound much more natural and far less artificial w/high def midi. I also made a patch with Max/MSP to apply high-def velocity scaling. Actually, the really interesting thing about this (eeeeee .... Pianoteq devs!) is you can accept regular MIDI from any keyboard and then scale it in Max to high-def. So this is something that could be built into PT so normal-res keyboards could at least scale onto high-res curves. Results are best when the normal unscaled diagonal line transforms into various curves - that lets the velocity values that come from the Privia map to a more nuanced range in PT. It's my opinion that the PT devs could do A LOT more with high-velocity resolution, in terms of velocity curves and also with random schemes for high-def velocity offsets. These kinds of things are valuable for high-def AND for normal keyboards w/out high def. On the other hand, maybe that's why things such as Max/MSP exist ... because it can be done in that environment as well.
Having said these good things about high-def, I've also found that it's not always as noticeable or as prominent as I thought it might have been. This could be because of the limits of what we can hear in terms of gradations of dynamics - or it could be that the however the high-def values are measured, it's not done in a particularly clever or useful way. The Privia transmits the high def values through CC#88. So, in effect, each velocity increment in the normal scale from 0 to 127 subdivides into another 128 increments. When PT is receiving hi-def info, the velocity values in its MIDI window change from integers to floats (resolved to 3 decimal places).
I tried to get more information from a piano world forum about how the high-def measurements happen on the privia. Unfortunately, at that point when I asking, there were too many PW posters with no experience with the Privia who nonetheless wanted to quote this and that about decibels, etc. to say humans ears can't possibly hear the gradations that come with high-def velocity and it's a marketing ploy and etc., etc. One poster there even told me that the high-def capability wasn't listed in the Privia manual (it's not) and so therefore it doesn't exist! (Well it does .... ! )
In any case, I've found high-def velocity curves to be the most useful use of high-def values. They definitely seem at least to me to let PT access a greater degree of nuance than regular non-hi-def curves.
Actually, I'm really only at the beginning of figuring out how to create the best velocity curves in Max (high-def velocity values going from the Privia going to Max and getting processed there and then sent out to PT). It's definitely one of those "the more you experiment the more you find" kinds of things ... For example, I'm seeing that some curves might work better with an offset, some might work better when multiplied (scaled) as the final step and some might benefit from random (hi-def) offsets. It might be too that there are times when I'd want to switch velocity curves on the fly depending on what's needed at a particular moment for particular passage. Well, this is all stuff that can be experimented with in Max/MSP ...
One last anecdote ... which is the Privia replaced a Numa Nero I had purchased a month and half earlier. When I purchased the NN I assumed all the quality-control issues you can read about on the internet HAD to have been addressed and fixed! I mean, most of the quality-control stuff you read about the NN goes back at least several years. AND, there are for sure a number of PT users who've had excellent experiences with the NN. That said, each time the NN went in for a repair (three repair trips in total) it came back with a different and new problem. And this was after the first motherboard was replaced! So, that how the came to be my NN replacement. Anyway, this long anecdote just to say that i retrospect I do very much appreciate that the Privia 4 times lighter or so than the NN.
Hope these comments are useful. .... I've been waiting to hear if anyone else has been trying or using the Privia ...
