I've been running ICS on my Nexus One for several months, this is my primary phone. Since neither Google or CyanogenMod support the device, I've tried a few of the 3rd party firmwares, by far the best IME is by dk_zero_cool
The only niggles that remain are camera related (no video recording, barcode scanning or panorama mode) and I believe a fix is pending. Sure the 512MB of memory and internal flash are tight, but it can use mount points to shove caches and app out to the microSD card.
I do wish that the work from all the XDA developers (and others) would get adopted back into CM9 proper though ... perhaps it will eventually.