There are a few reviews of the regular, and kingsized Bulkfillers, but to my knowledge there is little reference to the slimline on the English speaking boards. So below I submit my opinions on this writing instrument:
When I first heard that there was a new filling system on the block I was immediately intrigued. The bulkfiller mechanism is reminiscent of the telescoping piston mechanism used in vintage Montblancs.
For those that don’t know: The telescoping piston mechanism differs from your modern piston fillers in that the volume of all the moving parts is reduced by having the stroke recess into itself, aka telescoping. A smaller footprint in the barrel results in a higher ink capacity. A true mechanical marvel. These pistons were typically made out of brass giving those pens a very nice heft. Fountainbel has a very nice schematic of this ingenious filler here.
In fact, I think that comparison is quite appropriate. In my humble opinion the telescoping piston system was the best engineered ink intake system up until now. Conid’s website has a nice video demonstrating how their filling mechanism works. It is really simple to use. The new Bulkfiller just slightly edges out in front due to the ink shut off valve and double reservoir. My apologies for my bluntness, but I am trying to give really high praise here.