It's well worth the $40 for the app compared to higher end data acquisition units. The benefit is very few wires and modules to attach, and it integrates with iPhone's video capturing capabilities. The downside is the interface is difficult to use (by iPhone standards) and the data is strictly GPS and internal G sensor of the phone, the second of which isn't the most accurate (I think) in resolution.
The biggest limitation is that you have to view the data on your iPhone, but that's part of the allure I guess, you can see velocity traces and lineal and lateral acceleration graphs per distance laid out on sections of the track without having to download the data first to a laptop. You can also export the data to spreadsheet if you would like to run some detailed comparison.
Overall, it's good for DEs. Higher level and improvement via data will have to come from better DAX units. But for what it is? It's pretty good.
Here's an overlay video from last Friday: