Extra fine, with Noodler's and I get a nice fine line in my Moleskine (I use one of the bigger, squared ones as a personal Method's book, waterproof ftw!) The only problem is that I need to let the ink dry for a minute or two after filling up a page in the moleskine, and on every thing else it works fine and dries quickly.

Also, it doesn't work on the cheapo recycled paper pads my lab buys, bleeds like you wouldn't believe. Other than that, its great, and it's the only pen/sharpie/pencil that never wanders off cause the fountain pen part scares my labmates.