Originally Posted by
Rattus79
It will depend upon use.
For a big stopper I use a Singh Ray Vari ND $400 price tag, but doesn't colour cast and only vignetts a tiny little bit at less then 14mm on my 1.5 crop Pentax
For general landscape stuff I use the Cokin P series which does vignette at anything under 14mm. The cokin filters also tend to leave a magenta cast on photos. That is easily fixed in photoshop though.
A graduated ND must be slip in, otherwise it limits the creativity. Some of our best landsapers dont even use holders. Dylan, I'm looking at you.