what's the setup that requires step-down again?
It's useful when you like to vape at lower wattages, below 7 watts or so. For example, if you're using single, horizontal-coil low resistance stock atomizers like Kanger PT, eVod, T3S or LR cartomizers. Newer stock coils are either dual coils or single vertical coils, which can take more wattage.
I remember the original istick fired high/was inaccurate and needed either a higher build or lower watts.
The main problem with iStick 20 is that it's miscalibrated; for example, even though your display shows 3v, the battery outputs 4.2v (fresh off the charger), which is really annoying and can pop your coil if you're no aware of what's going on.
Watch pbusardo's review of the iStick20 if you want details.
But the D2 is a yihi chip, very accurate
Those are accurate--they tell you the lowest wattage they can supply, usually 6 or 7 watts.
Is it if you like a 10watt or less vape and only use the factory made coils at .5/1ohm?
Depends on the coil. Subtank .5Ω coil can take up to 25 watts or so, Kanger Protank 1.6Ω coil is designed for 5.5-7.5 watts.
