Documenting your code allows Visual Studio to pop-up the documentation when your class is used somewhere else. Furthermore, by properly documenting your classes, tools can generate professionally looking class documentation.
Write the documentation of your type with other developers in mind. Assume they will not have access to the source code and try to explain how to get the most out of the functionality of your type.
Following the MSDN online help style and word choice helps developers find their way through your documentation more easily.
Tip: The tool GhostDoc can generate a starting point for documenting code with a shortcut key.
If you feel the need to explain a block of code using a comment, consider replacing that block with a method with a clear name.
Try to focus comments on the why and what of a code block and not the how. Avoid explaining the statements in words, but instead help the reader understand why you chose a certain solution or algorithm and what you are trying to achieve. If applicable, also mention that you chose an alternative solution because you ran into a problem with the obvious solution.
Annotating a block of code or some work to be done using a TODO or similar comment may seem a reasonable way of tracking work-to-be-done. But in reality, nobody really searches for comments like that. Use a work item tracking system such as Team Foundation Server to keep track of leftovers.