The CMS I use, Umbraco, creates friendly URLs out of the box, which is nice. It further allows me to use either .aspx extensions for all pages, or no extension at all.
So I would get
What do you think is best? Personally I would prefer extensionless URLs for the categories and a plain .html extension for articles. The only reason why is because I used this approach before and Google seems to like it.
What is your take on extensions? Are they important at all? Do you think I should invest time in a different approach than either of the ones offered by my CMS?