Just my experience, but I bought one of those compression kits and the gauge was way off. It actually scared me half to death. It gave me 120 psi on every cylinder. I allmost threw in the towel. After doing some research I got another tester from craftsman and my compression shot up to 160psi. That's a 40psi difference.
For any tools required to measure important aspects of a motor, I would look elsewhere for sure. I have however had a 1/2 ratchet to some time along with lots of sockets from there.