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

This is correct it is designed to be the next update from PR1.3 this is why when PR1.3.1 was released CSSU incorporated it.
Using the opportunity, also changed there version labelling too IIRC.
Currently though the stable flavour features are limited to what I mentioned previously.

The update is designed (and rightly so IMO) so you see no change to the way you use your N900 before installing CSSU.
In the testing versions it seems many features are not optional, so they are tested.

If you wish to use the extra features of stable CSSU, other applications in the repo can provide a GUI for that. Or if you understand how to via CLI.

While I have seen claims of nine days on the N900 with the radios turned off I go from 100% on a brand new(charged twice) Scud battery to 37% in 13 hours with all radios off and almost no CPU usage.
With GSM and bluetooth on and running panucci audio book over bluetooth headphones for 50% of six hours I also end up at 40% battery.
I have CSSU testing installed and assume that tha would give me the best battery but I have been having this short battery life for months now and it makes the phone almost unusable except at home or office where I can plug in.
What can burn power if the radios and CPU are not the issue?
I am sure there is a tweak I need to use but just dont know how to activate it.
Below is the graph first showing 6 hours with three of panucci audiobook over bluetooth, the second is after a full charge and left unplugged overnight nothing running but battery graph.

try in xterminal by being root this code
apt-get install mp-fremantle-community-pr
hope it helps ..
cheers

Worked for me ... now i have cssu installed,same problem which u had i have .. @skip0pot2mus ... i flashed then now i installed but it cudnt install and now i did the code the it works !!