I've upgraded to the new Aperture 3 and noticed that they've added the ability to save and apply your own presets. I understand roughly that presets are basically a saved set of adjustments meant to speed up your post-processing work. Thing is aren't they created based on average cases and thus the adjustments won't be ideal for different photos? Based on this why should I use them versus doing it manually by myself? Are the time-saving worth the compromise (assuming there is) in the end result? How many people actually use them and how frequently?
Yes! While a preset is going to adjust for a common average and will not be perfect in most cases, they can get you close, and can be seen as a starting point.
Take for example a wedding photographer:
To greatly simplify things, you could say that there are a few basic shots that you take very often:
You could manually adjust each image, and you will get pretty good at making all the adjustments and knowing what it is you want... or you could set a preset for each situation, and apply the preset when you encounter it.
If you treat a preset as a starting point, and not a final adjustment, then you will see the huge benefit here. Instead of making the basic changes that you would make for every outdoor shot of a bride and groom as they leave, you apply the preset then make a few minor adjustments for the specific image.