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Originally Posted by Taallyn

There are screw-in Neutral Density Gradient filters. Granted you can't adjust them vertically in the same was as you can with the Cokin style, but they are made and used by some professionals.

Screw-in ND grads are mostly worthless. Most are made so you have the horizon directly in the center of the picture. It's usually a big no-no to have your horizon perfectly centered in your composition. Even the ones that are offset are very hard to work with as what you really end up doing is compromising your composition to use the filter versus using a filter to enhance your composition.

Cokin P filters are the way most professionals use ND grads. They are much more versatile. You don't need a perfectly flat horizon to use them either. They are sold with either hard or soft transitions. If you are shooting a sunset on the ocean, you can use a hard one that transitions very fast...more like a solid line. If you are taking a picture of a sunset behind the mountains, you use a soft transition ND grad. It will blend in very well with an uneven horizon.

Bear in mind that ND grads are not plug and play. Many folks (myself included when I started out) tend to think, "hmmm, pretty easy concept. Throw on this filter and "bam"...better pictures. Then you get frustrated because you can't get the results that Galen Rowell got. You need to understand how to use them and also when to avoid using them. You also have to understand that the filter won't improve your composition...only enhance the exposure a bit.