I've got it!
Last Wednesday I received a FedEx parcel containing my shining new TWSBI 580 fountain pen with B nib. It's my third TWSBI, after a 540 with 1.1 stub and a Mini with M nib.
I really like it, it feels great in hand... bit it feels almost the same that the 540 too. As I love my 540, that's not a problem for me, but if you look for a different writing experience, the 580 won't do it, it's clearly a new, improved version of the 540, as the 540 was of the 530.
A superb pen for me, the,. The B nib is juicy, generous, wet, and it writes like a dream. The Noodler's Upper Ganges Blue is beautiful in the transparent barrel, and the B nib allows to admire its nice shading on the paper.
After having used it since Wednesday, the 580 has become one of my favourite pens. The B nib is a bit too wide for daily use, but it is so smooth that I use it nevertheless.
In brief, a great pen, TWSBI!
As usual, I've posted this review also in Fountain Pen Network.