UPDATE: After two years, I still love this app and use it regularly. I've recently begun using a second body on occasion, and the tags can be applied to images from both--another advantage over a dedicated DSLR GPS attachment!
The developer is a really great guy, who responds quickly to bug reports and future requests. He also follows up to let you know when a version that fixes your bug has been released on the App Store.
One thing I will add, I've stopped using the interval updates and now I always use continuous updates. With interval updates, I am more likely to get random points in my track that are off by 1/4 mile or so. I think this is a GPS problem and not the fault of the app because other apps that use the GPS behave similarly when tracking is first turned on. With continuous tracking, this is not an issue (although the GPS can sometimes make errors inside or near buildings or under trees).
My original review follows:
Awesome! A $5 app that replaces a $200 GPS unit for a DSLR! It takes about 2 minutes to tag a days worth of photos. I shoot RAW, so I love that it creates XML "sidecar" files with the location information and it doesn't modify the RAW files. Then when I import the photos into LightRoom, LightRoom automatically reads the locations and can plot them on its map.
I've seen reviews that complain about the GPS performance, but that's an iPhone limitation and not the fault of the app. Anywhere the iPhone's GPS has difficulty (e.g. indoors), the app will too.
Like all GPS apps, it uses a lot of battery. You can save battery by setting it to only update every 3, 5, or 10 minutes instead of continuously. I wish there was an interval between continuous and 3 minutes, like 1 minute, which I think would be better for walking.