Fluorocarbon lines have been used by fly fishermen for a good while but have been adapted by bass and other fishermen fairly recently.  This line is made by combining a carbon base and other materials to form a line that is almost invisible underwater. Therefore it is very useful in clear water.

Many fluorocarbon lines are stiff and keep the shape of the spool, especially when cold. This can cause casting problems, especially on spinning reels.  The lighter versions show these problems less than heavier test lines but can still be a problem.

Some fishermen report that these lines are also are brittle and break easily.   Brands vary greatly and some are much better than others.  The best way to see if they work for you is to try several brands and test them under the conditions  you fish.

I keep spools of 12 and 14 pound test Vanish fluorocarbon line in my tackle box but use it only for leaders on Carolina rigs.  The stiffness does not create a problem in that use and I have not had any problems with it being brittle, even after carrying spools for several years.  

One experience convinced me using this invisible line makes a difference. During a tournament in a clear water lake, my partner and I were fishing a point with Carolina rigs. When I caught five or six bass and he did not get a bite, I gave him some of the worms I was using. The pattern continued, I caught several more and he did not get bit.  

After looking at his rig I noticed he was using a blue tinted line.  I gave him the spool of Vanish and told him to change his leader.  After making this change, he started catching more fish than me and soon filled his limit, too.  Coincidence, or did the line make a difference?  I am convinced it was the line.