Unfortunately, while the tools work well, the app doesn't actually provide any instruction on how to use them. If you don't know how to use a caliper or a level already, you're going to be on your own -- the app doesn't have a lot of polish or documentation, which is too bad. It'd be great to have a Tapbots-level version of an app like this, with as much thought given to the interface as to the functionality.
Still, Dimensions will provide a serviceable ruler, measuring tape, or any of the other tools when called upon. It's available in the App Store right now for US $1.99.