Here is a good introductory article for the SharePoint Chart Web part. I have not see this web part used that much, although, there is one feature that I think makes this data visualization vehicle critical for certain types of reporting. I am of course talking about being able to show a Sales Pipeline Funnel like this:
Few pros and cons to mention. Starting with cons, we have yet another tool in our tool box of data visualization technologies to consider (as if the existing list is not long and/or confusing enough as it is). The positive is simple, we can build a funnel!