We get this questions frequently - Do we prefer to use GND filters in the field, or high dynamic range (HDR) processing in Photoshop? Our answer is always pretty much the same. The important thing is to understand the advantages of each option, and to know when to use one of the other - or both.
Each option has it's place. We teach our students how to use GND filters during our on-the-ground workshops... and we teach our manual HDR blending techniques in our webinars.
These two images of Mount Adams Wilderness in Washington were taken to show a quick comparison. Jay used our manual "Intelligent High Dynamic Range" technique - dubbed iHDR somewhere along the line. He blended two bracketed images using layers and masks. The goal when we're blending images is to preserve the natural shadows and all the detail in the highlights. In the end, we want to create an image that is as close to our memory of the reality of the scene as we can bring it.
The second image was taken in exactly the same spot - just a few minutes earlier. For this shot, Jay used a 3 stop Graduated Neutral Density (GND) filter. The filter successfully brought down the highlights in the sky, so that the camera could capture the entire dynamic range in just one shot. You may notice that the trees at the base of the mountain are much darker in this image. That's a result of the use of the GND filter.
In this case, the GND filter worked well to reduce the brightness at the top of the image, but it also darkened the trees more than we'd like. For this particular image, the bracketed and blended shot is just a bit closer to the reality of the scene that evening. In the end, it's all about understanding the options that are available. Take the time to experiment with GND filters, and HDR processing. Find out what works best in any given situation - and most importantly, make the effort to understand WHY that approach is best.