Pay off student loans or invest?

Here is a link to a good article - with some interesting discussion in the comments - about a young graduate who wants to know if he should invest or pay of his student loan debt.

Read the article here. 

My preference in the case mentioned would be to concentrate on paying off his debt as rapidly as possible for a few years any way but since we don't know what his job is, or his future prospects, I see far to many people who either neglect to deal with their debt early and find it greatly restricts their flexibility in future years or who take on additional debt (mortgages, car loans, credit cards etc.) without a plan or any room for error. I'm a strong believer in only carrying debt one can manage.