Summary: If you shoot in sandy, dusty or rainy conditions it would be wise to protect your lens investment. I shoot quite often in an enclosed area full of marble dust and would not even consider shooting without a protective filter. Filters are much cheaper to replace than a $1,600 lens once you do get a scratch .
Summary: I always use Hoya filters on all my lenses. They are well priced and great quality. I have never had any problems with these and as we all know...protection...protection...protection. Specially if you use the L series glass.
Pros: Clear Optics, Consistent Effect, Firm Mount, Strong Construction
Summary: This filter is a terrible piece of equipment. Smudges are impossible to remove (using either a micro-fiber cloth or a LensPen). After a few minutes of cleaning, it develops a faint cloudy haze on both sides of the filter. Certain little spots on the coating (seen in glare) couldn't be removed at all. It also attracts dust. I have not had this problem with Canon or B+W filters.