I am by no means an expert, but figured that I'd share some of my thoughts and questions about this pen. I currently have a Sailor Pro Gear Realo (EF); a Pelikan M400 (EF); a Waterman Phileas (EF); 2 Hemisphere (F); and a couple other writing tools for special uses. Recently I added a TWSBI 580 to the collection.
I ordered one in EF from Goulet. Found the fit and finish to be much better than I expected. While it is not a pen I would likely bring to a jury trial or state supreme court argument, its fit, finish, and appearance are good enough that it has found its way into court with me a few times. For its price range, I'd give it an 8 out of 10 for fit/finish. In some respects it's actually better than my Sailor (removal of casting marks) and my Pelikan (easier piston movement).
I like the diameter of the pen, but the pen balance when posted gives me some issues. Back shifted a bit much for me. Also, pen feels a bit long when posted, and I don't have tiny hands. Only FP or liquid ink filling RB that I have that I write with unposted.
Ease of taking the pen apart is a big plus. Makes it easier to clean.
Have tried Jentle Epinard, OMBB, Kiwa-Guro, and Platinum Carbon. Got pretty good results with the Epinard. Had/have pretty bad nib creep with the Platinum Carbon, which has performed well for me in other pens. Has anyone else had that issue? OMBB gave me the expected nib creep, and some ink flow issues.
Kiwa-Guro had some ink flow issues but otherwise did well with the 580. Noticed some odd interactions between the ink and the plastic walls; instead of clinging to the walls, the ink would act like mercury in a vial and show no affinity whatsoever to the plastic walls. Sort of like what my sample of Kiwa-Guro did inside it's vial.
Has anyone else experienced flow issues with heavily saturated inks and/or "nano" inks in a 580 EF?
The nib is a little on the stiff side, but feels better on the page than my Waterman pens do. Provides a tight fine line which is nice for writing on forms, etc.. No noticeable scratchiness from the EF, and felt like it had a shorter "break in" time than my Sailor EF. Of the stainless nibbed pens I have, will say this one writes the smoothest. For being a stainless nib, I'd give it between a 7 and an 8. It actually writes smoother than my Sailor EF.
My overall impression is that this really is a pretty decent pen. Have not had it long enough to do any long-term analysis. I would recommend to anyone seeking a good everyday pen with much better ink capacity than a cartridge/converter filler. It may not be the flashiest, but it puts ink on paper, and sometimes that's all you need.