I have been running Cleanrom 6R2 for a couple weeks now, on both mine and the wife's. We had been on Viper, but both of us were having trouble. Her proximity sensor would not work correctly, resulting in "face dialing" going on every call. I tried a couple apps to adjust it, but it never worked right on Viper for her. In addition to that, I would see many random crashes... usually only after weeks of running Viper. But it would seem to get worse for the both of us, the longer we ran it. The more Venom tweaks I used, the poorer my performance got.
Cleanrom is based off the actual ATT build, and it has run better than anything else so far. It is very plain at R2 release... basically just ATT debloat + root. I miss all the customization, but the quality of other things is better. I can actually see butter, for the first time, on a few menus in this build. I threw Nova launcher on there, and the only real things I am missing right now, are better than the Sense status icons and hacked tethering. Dev still says he is going to release a final v6, which if as before will still be way more stock than Viper, but will have the big ticket things back (tethering).
If you are having odd issues however when running non-ATT software, give it a shot to see how it works. You cannot flash it using the latest TWRP however... need to downgrade TWRP to 2.3.x... not sure what is going on with that. I used the flashgui app to flash old version of TWRP I had in my goomanager folder.