Yes I have heard it should not be enabled. I have done an upgrade with cssu devel repo. First time there were just two thinga which caused problems (I mentioned them in my "Devel upgrades" thread) and they were easy to fix. Tried the same last week with fresh flashed N900 and made it not to boot anymore. Felt a bit sad. Flashed again (5x in 24h cause was messing around with trial and error) and did not make cssu devel upgrade, only with Extras devel I think.