I have an S3 with CM. The whole process took about an hour, there are guidelines everywhere, but make sure you download everything, including Gapps, before you get started.
Camera works fine in movie and still mode, and lots of options exist. The buttons require long presses to access functionality that I thought was lost when upgrading.
I unplug my phone in the morning, use it for tracking my ride or run with Runkeeper while playing MP3s on PowerAmp. It generally plays MP3s all day long, often while I check FB using Chrome.
Zero problems with phone and text, I have called 911 from my car stereo no problem.
The only problem I've had is that GTA: Vice City doesn't work. For some reason, when you get into a car the screen freezes until you get out of the car.
Oh, and the Bluetooth stack is done different, so I don't get track / album data transferred to my car stereo. Apparently this is because CM is doing BT correctly, which is different than everyone else so it doesn't work. I still get audio transfer. The text did transfer before I flashed to CM.
It will run out of power within 24 hours with that usage.