I've liked Pro HDR since it was first released. The HDR technique blends two or more exposures to capture better dynamic range in difficult lighting conditions. Apple provided some competition by building in HDR with the release of the iPhone 4 and iOS 4.1, but I still like the control that Pro HDR gives me both before and after the pictures are taken.
One of the big drawbacks to Pro HDR is that the location metadata and other picture information was not saved in the combined image, so location and other EXIF data was not available. That meant the 'Places' feature from Apple wouldn't work. Now that's fixed. It was the biggest single complaint that users of the app had.
Another advantage to PRO HDR is that it works on the iPhone 3GS and the 4th generation iPod touch, as well as the iPhone 4. It means owners of the 3GS haven't been orphaned when it comes to shooting great HDR pictures. HDR works best for stationary objects like landscapes or posed photos. If there is movement in the frame between the two images, the process will fail. HDR isn't for everyone, or for every situation, but used wisely, it can create dynamic photos that would be difficult to get without some help from the HDR process.