Digital Photography School has a post titled 5 Situations When Manual Focus is Better than Auto Focus, in which the author listed five situations where manual focusing might be easier. I will try to extend on what is already an excellent post. 
1. Macro work
Many people find manual focusing during macro photography easier. There are several reason for this. First, the lens may hunt for focus due to poor light conditions. Auto focus may either completely fail or becomes inaccurate. Secondly the point you want to have the sharp focus may not be at the center or any of the AF sensor locations. With very narrow depth of field, focus-and-recompose technique typically doesn’t work well for macro photography. In this case, manual focus will help you out. Not only it prevents the camera from focusing on wrong part, but also it allows you to fine-tune the focus with the help of the DOF preview function. Remember the camera focuses and meters with lens aperture at its maximum opening and stops down to the desired aperture when the shot is taken. What you see is not what you get in terms of DOF.
In situations when the DOF is not very critical, and you can place AF sensor point right on the spot, you may find auto-focus work quite well with good light. If you are chasing ants or bugs that move erratically, you may also want to stick with auto focus when possible.
For best results, your should try to always use a solid tripod. If you are serious about macro photography, you may want to consider specialized setup such as a focusing rail. Instead of turning the focus ring on the lens, which also changes magnification on many macro lenses, many skilled photographers use macro focusing rails for macro photography. The desired magnification is first set on the lens, then the subject is put in focus by sliding the camera and lens in and out without turning the focus ring on the lens.
2. Low light
If the camera has difficult in acquiring focus automatically, it makes sense to use the manual focusing. However, in the dim light, the viewfinder is dim too. If you don’t have a good eye, stay with auto focus. Most modern digital cameras now have low light focus assist system via a built-in lamp on camera body, or internal or external flash unit. They work remarkably well.
3. Portraits
This really depends on what portrait types you will be shooting. If you want to get into the artistic type that only leaves small part of facial feature in sharp focus and let the rest blur out, go for it. An alternative approach is to auto-focus using selected focus sensor point based on the desired composition. Read More…
