I have tested the Voldyne 5000 and find the device quite ok. It shows the current state of the lungs, although of course not as accurate as a PFT. The device also only measures the air when inhaled, i.e. the lung volume. An FEV1 value is not measured here. Also I don't see a big advantage at least with us CFlers. With inhalation, PEP and other instruments I find a lot more effect. Personally I need the device very rarely.