That suckers cheap, I would question quality of any digital that is less than a magnetic ballast on price alone lol. I thought about buying a 400 that could go down to 250w but opted not too, at what stage would I use the 250 I thought? It would only make sense if I needed the 250 to cool off my room during hot months, thus sacrificing yield for quality. The idea of being able to dim is more appealing than practical I believe, don't get me wrong I was sold on the idea for a long time. The guy at the local store told me dims like the one you're looking at can wreck bulbs, because they do not change the wattage as the high end ballasts do, they just simply dim the light (similar to mood lighting in your house) he said this is dangerous. The higher end ones (like what he sells coincidently) from what he said actually switch to the wattage, one goes 400/250/175 another 1000/600/400, but they actually program to those wattages.
My advice if you're wanting to stay in the low hundreds go with a sturdy time tested Magnetic, hydra farms has a few different lines of Magnetic even switchable ballasts.