DigitalFotografi.com have published a review of Calumet’s Genesis 300B AC/DC monolights. On the web site you can also find test shots with various modifiers. According to the review, the range of power adjustment is tested to be around 3 stops, not than the 5 stops claimed by the manufacturer.
In my preliminary tests the linear scale did not hold true. Full power (60) read as 16 and 5/10s on my Minolta V meter. (ISO 200). From there the reading were; 50 F16, 40 F11 4/10, 30 F8 6/10, 20 F5.6 7/10, and 10 F4 4/10. A five step range with F16 at full power(60) should deliver F11 at 50, F8 at 40, F5.6 at 30, F4 at 20 and F2.8 at 10.
The light is delivering almost one and a half stops more light at minimum power than the setting would suggest. Which means its really only a 3 and a half stop range. This was with the 8 1/2 inch reflector that comes with the light.
Update: For a second opinion you can find another quick review here.