Processing multiple exposures into a single HDR (high dynamic range) image turns out to be more of a subjective process than I initially thought.
Have a look at the following image, which was processed using Photoshop CS4 and the “Merge to HDR” feature:
Not bad, really, at first glance. But notice at the right edge, about halfway down the image, how the arch coming in from the wall is completely blown out?
Now look at the same source exposures converted by Photomatix:
The highlights are not blown out, the tonality of the image is smoother, the reddish cast is reduced. All in all, the image is much closer to the intended look than what Photoshop allows.
Given that Photomatix allows me a lot more control to tweak the result than Photoshop, I think the money is well spent on Photomatix …