I understand what you want to say, searched for the moment I installed CSSU, around Aug 2011. It was probably the 20.2010.36-2maemo15 (30.05.2011) version and I needed some extra files to make a full portrait mode on my device.
With the next update I didn't need those files anymore and if I remembered correctly 2 updates later you could choose between stable and testing version, and I went for testing.
For me the portrait mode was the most important thing to have so the extras like backgrounds fixed in portrait mode
and the camera-ui is really great but I understand that an update doesn't mean a total new device.
Step by step there's an improvement with each new update so I do see the difference than before I installed CSSU.
Also compaired the two versions by the wiki pages, stable and testing and there's a difference,
so I really see now what you want to say about what difference it has made to your N900