In a nutshell, it lets you pick out what the ringtone should say. It can be just an announcement about who is calling, like "Your friend Bob is calling from Las Vegas" or it could be a bit more insulting like "Your dumb buddy John from Indianapolis is on the phone". Basically, it takes what you've written and converts your text to speech in a voice you have selected.
Here's how it works: Type what you want the phone to say into a field. Then choose a voice, like British female, quiet whisper, American male and so on. You can also precede the message with a sound, and there are about 200 to choose from.
When you're ready, the instructions you've given are sent to a server (you must have internet access) where the ringtone is rendered. You retrieve it from a special website, and drag it into iTunes to get it on your iPhone. There are no fees or subscriptions, and you can make as many as you want.
Admittedly, that's all a bit convoluted, but Apple doesn't allow you to create and save ringtones directly on your phone, hence the workaround.
It all worked fine, and I liked the ability to make very customized ringtones, or more properly, caller announcements. I didn't think the sounds were of very good quality, and many were distorted, so I just passed on using tones before the voice announcement. The voices themselves sounded fine.
Making the ringtones and retrieving them is straightforward but Apple makes it harder than it needs to be. If you like this sort of thing it is certainly worth US $0.99. Note that the developer has a bewildering array of different versions of this app. I thought the uncensored version had the most options.