Is a circular polarizer lens filter necessary for good photography? Nope. Is it a fun tool to have for those special moments when the opportunity arises? You betcha!
This was the second lens filter that I added to my collection for my Nikon D50 camera. Definitely check the size of the lens you plan to put this filter on; they come in different sizes (mine was a 67mm to fit a 18-135mm lens.) I have a skylight filter that I keep on my lens all the time for protection, and when I want to use my circular polarizer, it just screws onto the front threads of the skylight filter. The lens cap still fits with no problem, so it really couldn't be easier to use.
This type of filter is also available at different price points, which is determined by the amount of coating is on the lens. (The more layers of coating, the higher the price.) I got good advice when purchasing this filter; get the best filter you can afford. The more layers of coating, the less lens glare and ghosting you will get, so the better your photo will be- definitely worth it.
A circular polarizer is something you won't use everyday, but it does come in handy depending on what you are photographing. It will filter UV rays, which will give you less haze when taking photos outside. More importantly, it will get ride of the glare you get from non-metallic surfaces (like water, glass windows, etc.) In other words, if you are taking a photo of a pond with fish in it, normally all you can see is the top of the water because the glare off the water. With a circular polarizer, you will be able to see all of the fish in the pond, since you are taking away the surface glare. The color all look normal though, since nothing is altered in that respect.
So, even though I don't use this filter everytime out, I am glad I have it in my arsenal of photography tools for those time I have needed it. I would definitely recommend trying one.