A CPL darkens blue sky, letting clouds stand out, darkens foliage and cuts relections from water/glass etc. You can dial in the effect by watching it through the viewfinder as you rotate the filter. They can be very useful in the right circumstances, but it's easy to overdo it, and they reduce light transmission. I have one, but I only use it occaisonally. Why not shoot for a bit and then decide if you really need one?
Get a decent one if you do go for it. The best are very expensive, but steer clear of low budget CPLs, because they can cause more problems than they solve. Hoya/Kenko Pro are pretty good, and reasonably affordable.