Something that frequently comes up in online forums is the constant debate about which brand of filter to go with for larger lenses. The two names which tend to come up more than any other (at least in the UK) are Cokin Z-Pro and LEE.
The Cokin option tends to be slightly cheaper (£42 per filter), and this can be a big draw. LEE are the higher priced brand (£56 per filter). Both filters may be used in the same holder. So why the big difference in price?
For starters, LEE filters are made and dyed by hand. The downside to this is that there can be a waiting list in periods of high demand (The 0.9 ND grads are like gold dust around Christmas). However, this also means that they are individually checked, and very consistent. But there's a little more to it than that...

The images above represent two possible scenarios that will be faced when photographing with filters. Both images are of the same set-up, with two filters loaded into the same holder. Each photo has the same exposure, using full, off-camera flash. Manual white balance was set from the white background and fixed for both photos. The only difference is in the position of the flash in each shot.
In the photo on the left, the light was directed from behind the camera, as if the photographer has their back to the sun (135 degrees from the subject). Both filters appear identical, as they are both the same density of 3 stops light loss.
In the photo on the right, the light is placed at 215 degrees, as if the photographer shoots towards - but not
at - the sun. Under these conditions, some strange things start to happen with the Cokin filter. As light cuts across the filter, the whole filter appears to take on a magenta cast. This is a well-documented problem with the Cokin brand, but many photographers either do not see it, or are not bothered by it.
With an ND filter, I can see why this may not be a problem, to a degree, as they affect the whole image. The advanced white balance correction in many cameras will largely negate the cast, explaining why it doesn't bother everyone. However, when you use a grad filter, only a portion of the image is affected. This means that you can end up with a colour balanced sky and green frost. (This is what first upset me about the phenomenon). This becomes a litle harder to correct without degrading the image.
But not as hard as the reduction in contrast when compared to the LEE filter:

It's fairly obvious that in the example above, the Cokin filter does not cut down the light to the extent it did when the flash was behind the camera, and the writing is harder to read in the top (Cokin) portion of the image than the bottom (LEE).
This is a significant difference, and my main reason for choosing LEE filters. It's not uncommon for photographers to include the sun as a compostional element in their landscapes, particularly during the "golden hours" around sunrise and sunset. When I choose to do that, I want to know that my camera's taking in the scene as I see it: high contrast, and natural colours.