Manual is mostly good for shots where you have plenty of setup time, like landscapes or studio portraits. To try and use it for changing lighting conditions in sports for example is far from ideal.
That being said, if your camera has auto-Iso Manual is given additional flexibility and can be used effectively for almost all types of shooting.
Recently I have found that using Manual and setting a shutter of 1/200th or 1/250th along with the desired aperture (for a given lens) works well for keeping up with my 2yr old son.
I can set f/2.8 + 1/250th for example and then let auto Iso take over from there to ensure the proper exposure. This ensures that I am not getting changing shutter speeds that might result in subject blur.
For this to work though the shots have to be on the side of underexposure, because the Iso can only go up and not down from 100. One way to quickly fix this while staying in manual though is to increase shutter speed as needed.
So this is my preferred method for low light, and its not that ideal for outside bright daylight...
For bright daylight I would normally use Ap and set for the desired aperture for the attached lens (2.8, 1.8, etc.) and set manual Iso-100.
This way the shutter speed can scale to what is needed for the proper exposure, while ensuring that I get the DOF and subject isolation that I desire.
Av mode has its uses as well no doubt; I just dont use it that often.
Basically its best to use the ideal mode for a given situation, they are all good in their own ways and have their specific uses.
- Neil