Me personally don't use CSSU because:
- I care about vertical view, I use n900 mostly as a phone, but I heard not all applications support it, so for me it's still only partial support. It means for some apps I'll have issues properly displaying which knowing myself, could be a nuissance
- I don't care about camera improvements.
- I read somewhere in this forum, that switching from SSU to CSSU different set of patches will also be needed (so it's like abandoning stability of nokia release and truly venturing out into something new, with hard way of getting back).
- It does not fix few issues I care: ie: http proxy support in media player or http proxy in build in im-protocols (gtalk, gg)
- I'm affraid it can break something to a point of unusable device, and instead of using my phone I'll spend days searching how and fixing it.

Summarizing: I'm able to install few applications and be part of the testing for few projects. But I still would like to use my N900 as dependable device (my only phone). In last few months I instaled few apps which were broke/needed some additional effort (columbus, opera, sleepanalyzer, mobile hotspot...) I also installed new kernel and then noticed I irreco does no longer work with no solution. This tought me little more thinking before clicking install for everything new from testing repo.Usually upgrades of applications are not worth upgrade because added value is low and risk of breaking something might be high. And I believe CSSU is "big" thing (I see it as major patch to SSU), so big risk.

Above is point of view of N900 user who does not spend all his free time in this forum, just taking a peek once a week briefly.