a. It is harder to create as we need to control the HTML that has to be rendered
b. A single copy a single copy of the control is required in the global assembly cache which can be shared between multiple applications, so maintenance is easy.
c. They can be added to the toolbox in Visual Studio
d. Full visual design tool support for example if properties of the control are changed, they are reflected in the design view.
e. They are useful for creating a dynamic layout
f. All the above