This has been discussed in this thread:http://luminous-landscape.com/forum/index....50entry153950
A lengthy discussion on the same subject at DP Review site: http://forums.dpreview.com/forums/read.asp...essage=25080916
Here is my take: It is useful if you are shooting JPEG: it adds a different tone curve to open up the mid-tones and shadows. Not that you could not come up with the same (or better) curve yourself though. In RAW it does nothing, except underexposing one stop (by shooting as if at the next full-stop lower ISO).
I think Bot Atkins got it wrong by claiming that HTP impacts RAW too (he is admitting several times in the article he is not sure what is going on with HTP). His update (to the initial review) and examples prove nothing, IMHO. What he did is the following:
"... RAW images shot at ISO 200, "developed" with a -2 exposure compensation..."
He used the same method for both non- and HTP files, which appears to be logical, but this is what really happened in my opinion:
Since HTP exposures are already underexposed by one stop, developing them with the same -2 compensation resulted in non-HTP files being underexposed by 2 stops, and HTP files being underexposed by 3 stops. No wonder the 3 stops underexposure retains more detail in the highlights.