I only use Adobe RGB.
Found this interesting tidbit:
Even the best monitors struggle to display 98% of Adobe RGB. Unless the screen you are looking at cost in the region of $AU 7500 or more, then the odds are your monitor's gamut is only slightly larger than sRGb and about one third of the size of ProPhoto.
And why you should not use ProPhoto in 8bit:
Using ProPhoto with 8 bit files is digital imaging suicide! It is incredibly easy to introduce banding to a file in 8-bit ProPhoto. Nothing makes a digital image look crappy quicker than banding (except maybe the Unsharp Mask in most people's hands, but that will have to wait for another article!). ProPhoto is so large that having only 255 possible shades of each of Red, Blue and Green is a BIG PROBLEM! Quite simply the distance between tones is so vast you are almost guaranteed banding with only slight manipulations, especially if you try and recover detail in a slightly underexposed area of an image, i.e. the shadows.
And the argument with 16bit:
Converting from RAW into ProPhoto makes some sense, as you will likely not run into ANY significant clipping issues. Having said that, it is not actually common that your images will contain tones outside of AdobeRGB. It tends to be only image with extremely saturated colour (particularly yellows) that raise this problem. This is easily seen using the histogram in Adobe Camera Raw. Really, if your original capture does not contain tones out of the gamut of AdobeRGB, then there is no point whatsoever (and quite a few negatives) in using ProPhoto as your colour space unless you plan to artificially manipulate the saturation of your image to a considerable degree.