Amended review: I must change my statement about the new CON-40 piston converter. As another reviewer stated, the CON-40 does fit the Cavalier series of Pilot fountain pens and, although it does hold slightly less ink than a properly filled CON-20 aerometric converter, the ink level is visible with the Cavalier fountain pen. One note: the CON-40, like all other Pilot converters, is a friction-fit converter and it does not screw into the feed section of any Pilot fountain pen.
Here's another break-down of the currently available Pilot converters:
CON-20: aerometric 'squeeze' bladder-type converter
CON-40: piston-filled converter
CON-50: piston-filled converter
CON-70: vacuum-filled converter (similar to the filling system of the vintage Parker Vacumatic fountain pens)'

I’ve read the complaints, but this little converter works well for me. Fill, turn upside down, push the excess air out and fill again. No sweat.

The facts, according to measurements made by the staff of the highly respected Japanese pen magazine, Shumi-no bungu bako:
CON-20 converter holds 0.5ml
CON-40 converter holds 0.4ml
CON-50 converter holds 0.5ml
CON-70 converter holds 1.1ml

Although the new CON-40 fits in the Elite and M-90 fountain pens which, previously, only accepted the CON-20 converter, it is impossible to see the ink level in the CON-40 when it is fully inserted the aforementioned pens. I have never had any problems with the well-made CON-20 aerometric 'squeeze' converter and see no reason to use the CON-40 in those pens.
I have tested the CON-40 in a Pilot Prera and it works as it should. The tiny bead weights with the force of gravity move the ink correctly towards the feed nipple. However, since the superior CON-50 with cylindrical weight and greater capacity works well in such a pen, I see no reason to continue to use the CON-40.
The only reason to purchase the CON-40, in my opinion, is if Pilot stops producing the CON-20. I gave the CON-40 four stars since it is well-made and it works as it should, but if Pilot continues to produce the CON-20, the CON-40 is a misguided addition to the Pilot converter line.

With a Pilot Namiki Capless, you'll only be able to fill this about halfway (~ .2 ml) b/c of the placement of the ink intake. Too little ink for a pen I want to travel with. Much better off syringe-filling old cartridges.

A really good quality converter, however, it doesn't hold very much ink. If you are looking for ink capacity, I would syringe fill a cartridge.