ColorPerfect will probably give you the best results ultimately. It is what I use ninety percent of the time. However, it is more fraught with pitfalls than any other method. Here are some ways of getting screwed up conversions out of ColorPerfect:
1. Clip any of the three channels in either end of the histogram
2. Include anything that is not film in the image. For example, some of the holder or even a largish hole in the emulsion
3. Fail to ASSIGN the colour profile that matches your WORKING PROFILE in whatever your image editor is (I use PhotoLine for cost reasons)
4. Fail to go to Options and set the black and white clipping points in ColorPerfect to something more conservative
5. Fail to assign the right film type
6. Fail to observe the %white clipped box at the top
7. Fail to adjust the gamma and saturation sliders to your taste first and adjust the black level slider last
Because it is so finicky I do often end up doing a manual inversion if I have a problematic image. However, whenever I have successfully been able to use ColorPerfect it has always done a better job than my own efforts. In particular, it is brilliant at removing colour casts that shift from highlights to shadows (like the typical green brights/purple darks situation you can easily get into in manual inversions). Also, it is much better at retaining fine texture. However, for the same reason, it is more revealing of scanner limitations and sometimes a manual inversion gives a "smooth" look that is appealing.
For a very long time I battled with black splotches in my shadow areas when using ColorPerfect. I eventually found out that this was largely a result of not assigning a colour profile which matched my working colour space. I find the software useful enough that I most often even run my positive images through it. But it is certainly a punishing learning curve to get the best out of it.
For what it is worth:
A set of my manual inversions:
Manual Negative Inversion - a set on Flickr
A set of my ColorPerfect inversions of negatives scanned as positive:
ColorNeg - a set on Flickr
A set of my ColorPerfect processing of positive "slide film" scans:
ColorPos - a set on Flickr
Sam