For macro work CHDK is extremely helpful for the following. (Once again thanks to all developers)
To maximize DOF, by using smallest workable aperture (F/11).
To synchronize built in flash to highest workable shutter speed (between 1/500 to 1/2000 sec) to freeze movement. Higher shutter speed under exposes as built in flash is not that powerful for my setup. This seems odd as to why flash exposure is controlled by shutter speed instead of aperture. In small auto flash's flash output is controlled by the duration it is on, that is it may be between 1/250sec to may be 1/10000 sec. As the flash is giving full output at 1/250sec or thereabouts it does not freeze fast action. You have to hit and try to find best combination for your camera.
Instantly changing the focus point from hyperfocal distance to infinity (normally you have to keep two buttons pressed with your two thumbs). For macro this is required as FOV changes with focused distance.
RAW: for improving good shots. (I don't waste my time with poor shots).
Auto ISO: In the field it is helpful (Auto ISO range should be minimum say between ISO 50-200 to minimize grain).
There are other uses also like usb trigger which I have not tried. This will help in reducing the camera shake when pressing the shutter release. The shake due to this makes your subject out of focus rather than freezing action.
I hope this helps
Edit: One very important use I forgot to mention. This is aperture locking through CHDK. The aperture control dial of S3IS is placed in such a position that when I am concentrating on the subject through the view finder, I inadvertently change the aperture. This way I have lost many shots. When I have to keep f/8 for closeups, I do it through aperture override. The aperture gets locked and no missed shots due to reduced DOF by change of aperture through dial under the thumb.