The Fastraxx is a bubble set up, it works just like the hub gauges but with the different attachment points. Getting it solidly attached to the wheel is definitely part of the learning curve, and some of that depends on the wheel itself and what the lip/spoke design kind of looks like. If the lip is flatter it works better. Getting the right tension is a little fiddly sometimes but you can use a bungee. The RPF1's on my car have a pretty decent lip for attaching it though so I don't have to do that on my Duster at least. Reading caster is always like that, you turn the wheel 15-20° depending on the gauge, zero it, and then turn in 15-20° the other way. The bubble gauge on the Fastraxx reads the actual caster though.
It's not perfect by any means but with some practice it works fine. And since most shops won't touch my car with its adjustable SPC UCA's and tubular LCA's, and certainly won't set my -1° camber, +6.5° caster it's better that I got the practice.