I have the GTech Pro Competition and it performs as advertised. The ET and MPH info is optimistic but repeatable. The Dyno graph is very nice. LOTs of Data available.
Check out their website as it answers all of your questions and more.
http://www.gtechpro.com/
How close is the GTech to a chassis dyno? I know it *should* read lower since it is reading "road HP" which accounts for aerodynamics, but do you guys think it's fairly accurate?
Of course, if you're using this to quantify upgrade gains, you're fine since you have a baseline to compare on the same device.