Here's a side by side comparison between an HDR (high dynamic range) image and a single exposure.

The image on the left was created by combining three exposures using Photomatix Pro 3 software. Tone mapping was performed using the Photomatix tone compressor engine. I made adjustments for brightness, tonal range compression, and contrast adaptation. The image on the right is the best single exposure from the set. I've added a slight S-curve in Photoshop to help make it pop. I should note that I've still used my trusty graduated split neutral density filter for capturing this image. Without it, I would have needed to combine a wider range of exposures in Photomatix, and my single exposure would be completely unacceptable.

The HDR image looks much richer. Highlights are toned way down while shadow areas show much more color detail. I'm sure it would be possible to get similar results using Photoshop alone, but it would be much more tedious and time consuming. However, I do prefer the way Photoshop renders blue skies compared to Photomatix, not a big issue in this image, but as I've been working with Photomatix I've noticed some color shifts in areas of solid blue sky that I haven't particularly liked.