I have been using the TWSBI Diamond Mini for a few weeks now and for the most part it has been a very enjoyable experience. This is the first pen I have purchased from the TWSBI range. One aspect of this company that surprised me the most was that they aren't the easiest of pen to find and purchase if you live in the UK. They seem easy enough to find via American websites but I have only found one website in the UK that sells TWSBI.
The reason I chose the Diamond Mini was that I have a wonderful full size fountain pen (Waterman) so I wanted a slightly more portable fountain pen. I went for the Diamond Mini in black and on first receiving it I was immediately impressed with the build quality. It seems to be made of a plastic material but feels very sturdy. A unique feature of the TWSBI range is that they don't take cartridges or use a converter. They use a piston built into the design of the pen. All you have to do is place the nib of the pen in to your ink of choice and twist the bottom of the pen and this sucks ink into the barrel. I went for the Diamine Sapphire Blue. The quality of the Nib is really impressive. I think the nibs are manufactured in Germany and this explains a lot as the EF Nib in the TWSBI lays down a line the same as an EF nib in a Lamy. Another feature of the design that has really impressed me is that the cap screws on to the top of the pen and also on the bottom while the pen is in use. The weight of the pen doesn't suffer from this and I find the pen to have a good weight balance. It never feels top heavy with the cap screwed on.
As much as I have had a great experience with the TWSBI, there have been a few things that have been less enjoyable. On the silver rim that is on the bottom of the pen cap, they have stamped the TWSBI brand name on it. The problem with this is that for me it stands out like a sore thumb, it doesn't suit the flow of the design. Another potentially bigger issue is that there is a silver rim that sticks slightly out just above the nib. This can be uncomfortable while writing for long periods of time.
Overall I have really enjoyed the TWSBI and I am really glad that I bit the bullet and bought one. The best compliment I can give this pen is that it is part of my daily carry. A good rule of thumb if you are thinking of buying one and your not to sure what nib size to go for, if you have ever used a Lamy fountain pen before and enjoyed that nib size. The TWSBI and Lamy nib sizes are very similar in the line width they produce.