The results were pretty consistent. The Apple HDR feature was the most subtle. It improved the sky, but clouds seemed to be washed out. There's no question that photos under appropriate conditions will be improved by using the app. It is very fast at taking three back to back exposures and rendering a tone mapped image.
Pro HDR is one of the best of the paid apps. It's US$1.99. It reached deeper into the shadows, and did a much better rendering of the sky. It's not as fast as the Apple app, because developers haven't been allowed by Apple to have complete access to the camera software. Pro HDR takes 2 images, combines them, and tone maps them. Before you save you have the option to adjust things like saturation, brightness, contrast, warmth and tint. Pro HDR has 2 modes, automatic and manual. In automatic mode, the software adjusts exposure for the light and dark areas. In manual mode you select the dark and light points by tapping on them. Since Apple uses an automatic mode, I used automatic on Pro HDR. I made no further adjustments, and just saved the image.
TrueHDR also takes 2 images and tone maps them. It has no automatic mode, and it is quite a bit slower than both the Apple HDR, and Pro HDR at combining and rendering a tone mapped image. From the time I started the Apple app it was 4 seconds until I had an image. It took Pro HDR 24 seconds. TrueHDR took a whopping 48 seconds, and does require user intervention to select the dark and light parts of the frame, and to trigger a merge of the images. I thought TrueHDR did a better job of capturing a sky with clouds than the Apple HDR feature, but the pictures were a bit too contrasty to my taste. TrueHDR is also $1.99
So is it worth it to still buy an HDR app? I think the answer for some people will be yes. While HDR in itself is controversial (some people absolutely hate it), it can certainly get usable pictures out of difficult lighting situations. If you have an iPhone 3G or 3GS, you'll have to buy an app to get HDR photos, because Apple HDR requires an iPhone 4.
This is very subjective, but to my eye Pro HDR did the best job of rendering scenes with high dynamic range. I think the Apple app was designed to not be as aggressive, so the results are subtle and pleasing. TrueHDR certainly gives a better sky in the conditions I tested it under, but it was a bit too contrasty, and the app allows no image controls before you save. Of course all the images can be cleaned up a bit in Photoshop or iPhoto, or any of the iPhone apps that allow you to edit your photos.
There's no clear winner, because taste about the 'look' of your photos varies from person to person. All the apps will improve over time, and doubtless there will be more features added. In my case, I'm hanging on to Pro HDR and True HDR, but I think Apple has done a good deed including the HDR feature to help people get better images from the already high quality iPhone 4 camera. If I was forced to use only one of the apps and had all three, I would choose Pro HDR.
Check the gallery for examples of all three apps shooting the same scene, and please share your experiences and comments.