Of all the filters available, the B+W 77mm MRC Circular Polarizer Filter will likely do more to add POP to your pictures than any other filter.
The B+W 77mm MRC Circular Polarizer Filter (as well as the other good circular polarizers) will cut the polarized light hitting the lens. The amount of reduction of the polarized light is adjusted by turning the end of the B+W 77mm MRC Circular Polarizer Filter. The effect is most noticeable at an angle of 90 degrees from the sun. Blue skies become VERY blue. Reflections are dramatically reduced or eliminated.
Be careful though - sometimes you WANT reflections and DO NOT WANT skies to be too blue! Like any other photography tool, use the B+W 77mm MRC Circular Polarizer Filter wisely.
Because reflections in atmospheric haze are reduced, circular polarizer filters can cut through haze to make outdoor pictures sharper. The B+W 77mm MRC Circular Polarizer Filter causes colors become rich and very saturated. Cutting the reflection coming off of foliage will remarkably improve landscape photography. This is an effect that cannot be duplicated in Photoshop.
Circular polarizer filters also double as neutral density filters. There is a light loss of up to 2 stops when using the B+W 77mm MRC Circular Polarizer Filter. This is good news for those really bright days when you want a long exposure, but bad news if you need a faster shutter speed or aperture.
Using the B+W 77mm MRC Circular Polarizer Filter at an ultra-wide focal length can result in an unevenness in your photo. Wide angle unevenness is especially noticeable in blue skies - the sky will be the darkest at a 90 degree angle from the direction of the sun light.
The above picture dramatizes the effect I am referring to. It was taken using the Canon EF 16-35mm f/2.8 L Lens at 16mm mounted on a Canon 1D Mark II body (1.3x field of view crop factor for an effective 20.8mm) with the CP Filter adjusted to maximize the effects. You have to decide if you like or dislike uneveness.
Although many brands of circular polarizers exist, the quality and performance of B+W MRC and Heliopan SH-PMC filters place them among the world's finest. Black anodized brass rings reduce binding or cross threading and to ensure optimal alignment. You do not want to put a cheap filter on an expensive digital camera and lens setup - your pictures will be degraded. I have not been disappointed with the B+W 77mm MRC Circular Polarizer Filter.
B+W also makes a "Kaesemann" ("encased") version Circular Polarizer filter. The Kaesemann Circular Polarizer Filter is completely edge-sealed for maximum durability under extreme climatic conditions. Lesser non-sealed Circular Polarizer Filters can suffer from foil separation at the filter edges when used in these conditions.
Since many of the lenses in the Canon L Series Zoom Lenses have 77mm and 72mm filter threads, I am able to share two of my Circular Polarizer Filters among most of my lenses.
Be sure to get a "Circular" Polarizer Filter if you are shooting with an autofocus camera body such as the Canon EOS bodies. The "Linear" Polarizer Filter interferes with autofocus of these cameras. A Linear Polarizer Filter can be used only on certain manual focus camera bodies with no problems.
In direct comparisons, I have a hard time seeing a difference between the B+W 77mm MRC Circular Polarizer Filter and Heliopan Circular Polarizer Filter results. If I was forced to pick one, I would say the B+W MRC might have the edge - but the results were very close.
Of specific note to B+W Circular Polarizer Filter users: The surface of B+W CP Filters looks mottled - almost deformed - when held at the right angle under the right lighting. This is normal. Here is B+W/Schneider Optics' explanation of the anomaly ...
It is a known cosmetic side effect (artifact) of the high performance polarizing foils that Schneider/B+W uses in its True-Pols to produce a "ripple reflection" to the human eye when viewing high intensity reflected light that "bounces" off the actual polarizing foil. This is simply a "reflected artifact" and is not transmitted through the lens.
This effect is apparent when viewing the filter under certain specific lighting conditions (such as a bright table light being reflected off the polarizing foil's surface from 45 degrees behind the viewer). It appears as a slight visual "rippling" appearance in the virtual surface of the foil. You cannot see this effect under any other viewing conditions. It is an artifact from a reflection coming from the polarizing film.
This phenomenon shows up in the visual appearance of the filter only and has no affect on the photograph (transmitted) image at all.
Some of my most striking pictures were taken using Circular Polarizer Filters. The resulting saturation is beautiful. Be sure to check out the sample photos in the link below. Please note that the sample pictures are intended to show results from quality circular polarizer filters in general - not necessarily from this specific one.