I still shoot landscapes with a DSLR, and one of the 5 filters I use most often is a circular polarizer (CPL) because it helps to filter out the scattered light rays that cause glare on water, windows, and wet leaves, and that wash out the sky's normal blue tint.

There's not really a lot to say about a filter, though--does it do its job? Yes.
Does it do its job well? Yes.
Does it do its job well enough to be worth the purchase price? Absolutely yes--selling one or two prints taken with this filter will cover its purchase cost in profit.
Does it make it easy to take landscape shots that will sell? Skill plays a large part in that, but this filter does help to limit a few sources of the glare that can (not always) ruin landscape shots. It's also good in light tents and other situations where you have multiple strong light sources from different directions.

It's multi-coated glass, so they've paid pretty careful attention to limiting how much light this filter allows to bounce around, and to making it pretty much point-and-shoot once it's installed on your lens, even if you're still using the green box (full-auto mode) on your camera. This is my 4th CPL filter in different lens diameters; I've never had one be so great or so bad that I had strong feelings about it either way.