I bought this pen partly out of curiosity, partly purely because it came in a mint green colour and I love mint green.
It's relatively cheap and to be honest, it looks and feels cheap. When I hold the pen I can feel the joint between the nib section and body move slightly.
I will be sticking with cartridges for this pen, as the filler is likely to be pretty teeny; indeed I haven't read a good review of the bottled ink converters.
I opted for a medium nib, as it's my preferred width. As with my Parker Rialto, this medium has a slightly flat appearance, so there's a discernible difference in stoke width. To my mind, that alters the look of my writing - and not necessarily for the better.
The nib is ok - I have had some skips and false starts. It isn't nearly as gorgeous as the nib on the Faber-Castell Loom. As said earlier, I don't think it quite suits my writing style.
I have used two ink colours in it so far and it doesn't seem to produce the colour depth that some of my other pens do. Diamine's imperial purple looked a bit more like a pale violet than the rich depth I get when using it my Faber-Castell.
The pen is as light as a feather and teeny when closed. It needs to be posted, or it is a bit short for writing with. After several days of usage the nib still skips a bit. Other than that the pen is easy to use. I have been writing with it quite a lot, to see what I think of it. For jotting down quick notes it is fine and for a smaller hand it would be a great pen. For me, the barrel is just a little narrow to be comfortable for a long writing session, as I have large hands. I have a similar problem with my Parker Rialto and for that reason I prefer to write with my chunkier pens.
These Kawecos get good reviews, for value for money and, apparently, for being extremely robust. I have no desire to test that, but it is small (and cheap) enough to drop into a small purse or bag.
In conclusion, this is a perfectly useable pen, good value for money and easy to chuck into a bag. That said, it's a little too small for my hand and I doubt I would buy another Kaweco Sport.