First up. It is a great pen and a pen I will recommend. I am very pleased with the weight and feel of it, and the way you can see the ink in the clear demonstrator is very nice indeed. That said. I have had mine burp in my pocket - luckily the cap held the spillage. Also the nib is a bit picky with angles. (or maybe it is a little overpolished...) but a smooth operator over all - and a joy to own despite.
What I like about Opus 88 demonstrator are: well design, good pen diameter size, contain a lot of ink with a classic eyedropper filling, and smooth writing. What I see that need improvements: The inner wall of the pen cap is susceptible to contact with nib and leave the ink trace, And it will be clearly seen since it is transparent. Ink may penetrate along the barrel thread which makes it look less nice. Once happened, it is not so easy to clean.
It is a great pen, it looks very different, and the thick grip works well for me. It takes a massive amount of ink, and the mechanism makes it easy to take it into planes without fearing a leak.
I ordered the broad nib, and like all schmidt nibs, they are very very smooth.