If these specs are accurate:
http://www.falconeyes.net/studio.htm then these lights are exceedingly inefficient. Th DE-600B claims 600W/S, yet only has a guide number of 84 for ISO 100 at 1 meter. Even if this were 3 meters they would be weak. Similarly, the DE-900B claims 900W/S yet has a guide number of 93 for ISO100 at 1 meter.
That means that if your subject was 1 meter from your lights, you would set your aperture to f9 3/10 to get a normal exposure with the DE-900B at full power.
To put that in perspective, an AlienBee B400 (160W/S) (
http://www.alienbees.com/b400.html) has a guide number of 112 for ISO 100 at 10' (3 meters). That's f11 2/10 at three times the distance. Or to put it another way, I would expect to see a 900W/S monolight have something around a guide number of 640 to 900 for ISO 100 at 1 meter (f64 to f90)
Personally, I wouldn't recommend these lights to anyone. You have no idea how easy they are to fix or find replacement parts for if something breaks. What is their big draw for you? If it's price, it has been my observation that anything cheaper than an AlienBee should be considered disposable.
-Chip