If you don't have a full time machine backup before installing make sure you have a backup of anything you don't want to lose forever made before installing...
It's very stable for me, and is much more responsive that Yosemite was on the same hardware. There are a few apps that won't work due to the new 'rootless' security but other than that I'm satisfied.
Just make sure you perform a full Time Machine backup before you make the upgrade. And by the way, the upgrade is 'in place' so you don't lose anything. One word of warning though, if you have unsigned kexts installed, El Cap will ignore them rendering them useless.