I was given a Silpat baking sheet as a gift.  As someone who has baked for years, I was skeptical that it was really necessary.   Boy, was I wrong!  For cookie baking it is indispensable.  Whether the recipe calls for greasing the baking sheet or not, I always line the pan with a Silpat.  The cookies cook and brown more evenly and are easily removed from the pan.  This is especially useful for thin, delicate cookies.  I've also used it for baking meringues instead of parchment paper. 

Some may feel they are a bit pricey at $20.00 per sheet.  However, given the time and money involved...