There's a couple. Some have been having issues with gapps and have been experiencing problems with internal memory being moved to /sdcard/0/0/0/0 which causes CWM to not be able to see the internal memory files. I'm not sure if it's still on going but you can check a couple out:
I got stock 4.2 on my Nexus 7 right now and it's great. Considering it just released for the Gnex about a day or two ago, I'd give the dev community a good week or two before they get everything stable. For now I'm sure we'll be seeing alot of patches to fix little issues.
There are a couple others listed at the link below that have been updated since the original post to be bug free for most users, myself included. I just want some Bugless Beast soon. I believe most of these ROMs below are essentially stock AOSP ROMs, without many (if any) extra customizations/features.