SpeedBox is an app that attempts to tell you exactly how fast you and your iPhone are travelling. It uses the A-GPS that the iPhone has available to pinpoint your locations and your speed, but unfortunately it's not accurate enough to beat a speeding ticket.
The apps description specifically says that it works best at speeds over 20km/h. That means that you're probably not going to find this useful as any sort of exercise app. Perhaps you may find this useful if your speedometer is broken and you want to get a general read on how fast you're going. But besides that there isn't really much else to do with the app.
It constantly updates, so even just messing around with it will suck down your battery life. At the bottom you'll see a GPS Accuracy indicator. That will tell you how well it claims to be picking up your speed, but even sitting still you'll see that the speed will still fluctuate from 0-3 km/h.
The idea behind this app is great, but the fact that the iPhone doesn't have true GPS means that you won't be receiving the most accurate of readings. That doesn't mean that there won't be plenty of future uses for this type of app. When you check it out, view it in the same mindset that you would when you first saw Here I Am.