My experience of HTP is that I don't use it. I used it once, in ignorance, back in Jan 2008 and never again since. No harm to the images since they were snow scenes and very, very bright. But I am now better informed and would not dream of using it, AS A RAW SHOOTER. If you shoot to JPEG then that's a different matter, but one which does not apply to me.
Those who have analysed RAW files produced with and without HTP enabled have determined that the RAW data from an image shot at 200 ISO with HTP is the same as the raw data from the same scene shot at 100 ISO without HTP. HTP "buys" you more highlight headroom by applying less gain to the sensor output. You end up metering for one ISO value and shooting at another. You are moving your exposure one stop to the left.
While the raw data will appear the same from either approach, with HTP enabled you will get an entirely different image and histogram on the back of the camera. You will be seeing the results of gain applied by the tone curve instead of gain applied to data from the sensor. If your goal AS A RAW SHOOTER is to expose to the right then HTP will undermine your efforts by showing you lies on the back of the camera. If you shoot to JPEG then no problem. What you see is what you get. With raw, what you see is not what you get, not at all.
Richard, you know full well the impact of picture styles and contrast settings on the "accuracy" of the histogram for raw shooters. Well add HTP to the list of conspirators. You can probably put it at the top of the list.
If HTP is so good, why is it even an option? Why not make it part of the standard processing? Because it robs Peter to pay Paul. You can't magic up extra dynamic range from the sensor. You can only fiddle with your processing of that data. If you want extra highlight headroom you have to sacrifice shadows. Canon knows that. The warnings about increased shadow noise are there in the manual.
I won't say that there are no advantages to HTP, because for some people I know there are, but for me, as a raw shooter who meters for highlights and does not use DPP there are none. Absolutely none. If you're shooting to JPEG on the fly and aren't in a position to nail your exposures every time then it may well be useful. In the studio, with controlled lighting and metering, no way, even with JPEG, unless you happen to especially like the tone curve it employs.