+1 on Neuro and EOBeav, I just want to add, that if budget allows, I think they could coexist in a bag nicely
e.g., dark ND can help you smooth moving water and CPL, while mostly mentioned with regard to the bluer sky effect, also reduces reflections off nonmetal surfaces like glass or that aforementioned water, and conveniently, there is a nice example posted here, today... look at the 4th picture in the OP
http://www.canonrumors.com/forum/index.php?topic=13480.0 , I think that that you can see the rocks under the water adds an interesting touch to the photo... and, an ND was used as well (so much to the coexistence

)
Btw, should you need more justification, you can use them in other than landscape photography as well, portraits would be more obvious, but I like CPL also for cars, and except the obvious windows it works on the body as well, because although it might be from metal, the clear coat isn't... thought that might be irrelevant to you