I had the opportunity to try all the sizes and finally got the medium one. In a short time, I tested it with some of my inks and here are my impressions so far:

1) J. Herbin Rouille D'Ancre
While this ink shades beautifully with this nib's impressive flex, it had pretty dry flow most of the time and I experienced some railroading.

2) Pilot Iroshizuku Fuyu-Gaki
As expected from an Iroshizuku ink, this one flows smoothly. Well, maybe a bit too smoothly. While there wasn't any railroading, I think this ink was actually too wet to be used in a flex nib like this with an ebonite feed. When used with no pressure, I had pretty thick lines with less precision where they were otherwise thin and precise. Still it is a lovely color and I haven't had any issues about bleeding when paired with Tomoe River 52gsm.

3) Parker Quink Black
This is my favourite so far! While it's not as wet as Fuyu-Gaki, this ink still flows well and I think this is the one that was able to catch a balance between too dry and too wet. There wasn't any railroading even when used in long sessions. Dries reasonably fast with a bronze sheen. Being classy and affordable to keep up with this "fountain" of a fountain pen -an ink drinking monster really-, this one was my favourite performer. Both on the paper and in the wallet.

As advertised. A very nice product.

The thick one is good, but the thin one is not very thin, so it's not very good. If this is the case, the G-pen unit would be better.

Just got this, so far so good. great customer service from Osprey Pens, and the nib is super fun to draw with - much easier to draw with than the zebra G nib (although that's great as well) because the tip is more rounded so it doesn't catch on the paper if you draw upwards - the line doesn't go quite as thin as some of the other nibs but the ratio from thick to thin is incredible, with no railroading, I'd guess about 0.2mm to 4mm, looks super cool. Thanks guys!

I have a lot of flex nibs, both modern and vintage, and my Osprey pen with this nib is one of the few I always keep inked because I enjoy writing with it so much. The most noticeable thing about it is how smooth it is for a flex nib. Even if I abuse it by pushing very hard upwards on rough paper, it does not catch. The other excellent feature is how strongly it snaps back from flexing. It's a reasonably firm flex and can flex 1.5mm ~ 2mm when pushed, but it takes a lot of pressure to flex it that far. If you use it for ordinary copperplate then it's an ideal firmness for good control. The nib and feed deliver a lot of ink so it has never skipped on me, but it does mean that inks come out quite dark. I tried a sheening ink in it and that looked really nice, but I don't know if that kind of ink is good for the feed. Finally, I have the EEF and that gives a very thin line with no pressure. Not the thinnest of any modern pen but easily good enough for a nice hand.