I had previously been using CM6.1.1 on my N1 for quite some time and was happy with it and the features it provided, but I wanted to update to something newer; with the CM team deciding not to provide ICS for the N1 Evervolv seemed like the best alternative option.
I tried it out on my phone and was overall very impressed; it ran smooth, (most) things seemed to work great, and I was able to size my partitions to have enough working storage to run ICS even on the megabyte-challenged N1.
In comparison to CM6.1.1 though, there were some key things missing that I found very frustrating, followed by a (for me) game-breaking bug that made me flash back to the older OS for now.
Firstly, a couple things that I really enjoyed were that the phone was switching seamlessly between home and roaming networks, something that's always been a struggle with my N1. The market works very well, apps install fast and don't seem to get jammed up at all. I had no problems with the wifi or camera, and I really enjoyed the modernized UI of ICS itself.
Things that were missing and I've gotten very used to having, though:
-No RenderFX widgets (allowing me to only light the red subpixels for battery saving and phone use at night without eye strain). This is a big one to me, and I wouldn't expect it would be hard to port it over from CM's code.
-No home screen power control widget, and the one in the status bar pulldown needs to inexplicably be swiped to the side to reach it (CM's is right on the main pulldown, in addition to having one for the homescreen)
-No dB display to replace the ambiguous signal "bars" - as an EE the dBm values mean a lot more to me and it's nice to know the actual strength
-No ability to skip tracks by double-clicking the trackball with the screen off. This also doesn't exist in CM7 and later for some reason, but it's extremely useful to me - when I'm riding my motorcycle with the phone in my pocket it's the only convenient way to skip tracks (reaching/feeling the volume buttons through jeans and gloves is nearly impossible, but the trackball is fine).
There were also a couple minor UI annoyances with the contacts list and browser, but these aren't Evervolv's fault as far as I can tell (they're ICS-standard) and they're probably outweighed by the other UI improvements overall.
The final straw that made me revert though was a bug vaguely mentioned earlier in the thread where the OS doesn't detect headphones being plugged in - it was played off as a hardware problem in that discussion but upon reverting to CM my headphone jack works just fine again. Even if this is intermittent, I listen to music on my phone frequently and if I have to reboot half the time to get it working it's not worth the trouble for me.
So right now I'm back on cyanogenmod (testing the 7.2 version right now and if it gives me trouble I'll just go back to 6.1.1). If Evervolv were to track down and fix the headphone detection bug and implement even a couple of the features I really miss then I'd be right back on it almost guaranteed. Running ICS on the N1 is a pretty great experience in general, if it has the features I need!