Auto tracing is great and almost instant, just a couple of clicks and you are done, so you can save tons of time and money, but automated tracing doesn't work well with all images. It really depends on the original bitmap graphic; the quality, the contrast between colors, even the size. I think the perfect candidates for automated vectorization or auto-tracing are images that are:
- Non-geometrical "free flowing" shapes - like the sample tree below
- Large, good quality originals that are black and white or high contrast colors with very clear distinction between each color.
- Photographs that do not need to be changed to simple line drawings
The images that will most likely need to be vectorized manually to have a good result, are images with geometric shapes, poor quality originals and images with color blends and gradients. See the difference between auto-traced vs a manually drawn vector graphic:
Samples of photographs vectorized by auto-trace
Samples of photographs vectorized manually and changed to line art
General vector conversion samples