It's a particularly large opening when it works, and even if it doesn't, it lets a lot of light into what can be a very dark interior (thinking mainly of the very common Soul Black colour scheme here), plus it makes the car look even more modern with a complete sweep of glass from windscreen to hatch. It's wonderful to watch in operation and you almost can't believe a small car like the A2 could have had such engineering marvels 20 years ago.
I've had an A2 with an OSS that worked properly, but I had no idea how long previously it had been fixed - it almost certainly wouldn't just have just kept on working from new! So I sometimes had that slight unease when I opened it, and when it rained. I would have had it serviced by
@Kleynie, except I ended up selling the car on to make room for a campervan.
My user experience was that, while it was great at letting the heat out of the car, you had to drop the back windows a couple of inches to ensure airflow and thus avoid the 'helicopter' wuthering noise (this is a lot easier with electric rear windows, but the mechanisms for those are also prone to brittle plastic failure in their advanced age). Even then it was noisy at much over 50mph with the roof open, and you could feel some effect of the extra weight on the top of the car affecting its centre of gravity the whole time. Sometimes it was just perfect to cruise along gently with the roof open, more often I'd open it while in town and close it down to the tilt when I got out onto faster roads.