Do you guys factory reset after flashing CM?
The best way to install is to flash Official CWM recovery from their website (Google Clockworkmod), download .img file and flash with SGS4 FLASHER from Play Store.
Boot to recovery
Go to mounts and storage
format system, data, cache. (make sure you've done a backup of current ROM before doing ths)
Then flash CyanogenMod ROM
Then factory reset
You should be fine
If you get stuck on splash screen it's cool, Just hold power down and try again. You may need to keep doing this till you see CyanogenMod boot animation. I've had to do it 4 times before it would finally boot. It's normal though just something weird with CM10.2 kernel. If you flash a custom kernel it boots fine everytime
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