I stumbled upon
HDR One magazine the other day and obviously it's a great resource for the type of photography that I most commonly do. As well as the articles themselves, it's also provided some great suggestions of HDR software out there. I've picked up a demo of
Machinery to try it out and it's definitely an interesting contrast with Photomatix. Machinery does profess an intent towards
natural HDR, which isn't usually what I aim for but maybe I should. All the more reason to experiment.
I must emphasise at this point that I have, at time of writing, only had to run a couple of images through it so there's lots more exploration to be done but here are two images- the first done in Photomatix, the second done in Machinery:

The difference is obvious- Machinery certainly does create a more naturalistic HDR image. Also look at how crisp that bastard is! Not only does Machinery provide built-in sharpening but the halo-ing effect (so often the bane of HDR) is reduced, presumably due to the more natural look. Take a look at the streetlights on the left of the image, for example (both in the foreground and the BG) and the superior detail on the shelf of Buddha figures. It's also astonishingly fast and does a genuine live change as opposed to Photomatix's
two-stage 'guestimate' process, which has always bothered me, to be honest.
Obviously this image comparison also depends on the settings and post-HDR-processing each image has been run through. They've both had the contrast lowered and brightness increased. The Photomatix image was sharpened a touch and the Machinery one had it's saturation upped a bit since lowering the contrast left it a little washed out (or maybe I'm just too used to Photomatix output).
Overall, perhaps as you'd expect, it seems that the different softwares both have areas in which they are superior but it certainly seems that the de facto industry standard position that Photomatix seems to enjoy can be challenged. In any case, I definitely need to do some more experimentation with Machinery to see what I can get out of it but first experience has definitely left a positive impression.