I recently have become very interested in the different filling mechanisms used in fountain pens. As a result, when the TWSBI Go was introduced with a spring-loaded piston mechanism at a reasonable (under US$20) price, I decided to order my first TWSBI.
The Go is made of solid-feeling plastic. I like that everything on it feels nice and tight. The pen is a bit short, and I find the width a bit wide for the length of pen. It doesn't really seem to be intended for use posted, which is how I usually write (unless the pen is Capless).
This is a demonstrator pen, so everything is on display from the spring that is responsible for drawing the ink to the large ink reservoir (making it easy to see what color is inside). it feels much more substantial in the hand then similar clear plastic pens.
I purchased an F nib, which seems to be appropriately labeled. Not particularly smooth, but not scratchy either. I also like that the pen is tapered towards the nib; it makes the width of the pen more suitable (at least for my hand).
Pros: LARGE ink reservoir, interesting and fun filling mechanism, price, seems pretty leak-proof
Cons: A bit inelegant, a bit wide, the nib is just adequate
Bottom Line: Though I like the filling mechanism and ink capacity of this pen, I don't expect to use it often. I did fall in love with the new ink I opened at the same time, however!