Perhaps more than any other filter, the B+W 77mm MRC Slim Circular Polarizer Filter can add POP to your pictures.
The primary purpose for a slim version of a CP filter is to prevent vignetting on ultra-wide angle lenses. Being slim is more important for a CP filter as the B+W 77mm MRC Slim Circular Polarizer Filter is about the same thickness as a normal B+W UV filter. I guess sometimes being slim is relative - and in this case it is indeed slimmer than the standard B+W CP filter.
I use a slim version of the B+W MRC Circular Polarizer Filter in part so that I can quickly mount it on top of a normal B+W UV Filter without causing vignetting (in most cases). Stacking a CP filter on top of a UV filter is not a great thing to do, but sometimes not-so-great is much better than not-at-all (such as in harsh conditions - such as salt water spray). Just to be clear - I don't recommend stacking filters for normal use.
Why aren't all filters slim? Good question - there are several reasons ...
First, a normal lens cap does not mount on the end of the B+W 77mm MRC Slim Circular Polarizer Filter. B+W supplies a snap-on (push-on) friction-fit lens cap to protect the filter. This works, but the cap tends to fall off easier than the standard Canon cap (it easily catches on things). Pictured below is the B+W 82mm MRC Slim Circular Polarizer Filter and its push-on cap.
The B+W 77mm MRC Slim Circular Polarizer Filter does not have front threads. The primary reason for these is to hold the OEM lens cap, but occasionally it is useful to stack filters.
The B+W 77mm MRC Slim Circular Polarizer Filter is more difficult to adjust because the adjustment ring is very narrow ("slim" or thin). Reaching under a hood to turn a CP filter ring is not easy - the thin version is even a bit more difficult to use. But, vignetting is even less acceptable to me.
The cost of the B+W 77mm MRC Slim Circular Polarizer Filter is higher - something I'm sure all of us care about.
According to B+W/Schneider Optic, a slim filter is required when the effective focal length (field of view crop factor included in calculation) of a lens is wider than 28mm. However, I have found this figure to be dependant on the specific lens. Stacking filters will create the need for a slim outer filter at an even higher focal length.
Using the B+W 77mm MRC Slim Circular Polarizer Filter at an ultra-wide focal length (where thin filters are most necessary) 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 B+W 77mm MRC Slim Circular Polarizer Filter adjusted to maximize the effects. You have to decide if you like or dislike uneveness.
Please read my B+W 77mm MRC Circular Polarizer Filter Review (non-slim) for more information about Circular Polarizing Filters. 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.