The nib is steel, but it is gold plated and the tip is iridium. The combination makes for a stiff, but smooth, writing experience. I bought two nib units for it, and both the Bold and Medium nibs are excellent. The Bold is a little wet with most inks, but if you're writing on good paper then you're going to love the shading that it puts down. The medium puts down a substantially thinner line, and it a fair bit less wet. Even pretty thin inks seem to flow well in it.
The only draw back, and this is due to the eyedropper-setup, is that you will occasionally get a drip from the nib. It seems to happen mostly when the ink is low in the barrel or it gets heated up while pointing down. I keep it in my pocket most of the time, but I take care to keep the nib pointing up, and I haven't had any problems. As you can see in the picture below, when you get a little ink in the cap of the clear model you can really see it. I'm sure I could clean it up, but it doesn't bother me so I don't.
I doubt that you would have that problem with a cartridge, but I can't really speak to that.
Goulet only sells one of these, and it's not the clear one that I have, so here's a link to the Jetpens site with the clear demonstrator version.