A very good reminder.
Beta software is NOT complete. It is intended for developers to test their apps (programs) with the new system before it is fully released.
It WILL be incomplete. There WILL have bugs (problems). There may be features added or removed at the time of the final release.
It may seem stable, but don’t put it on a computer or device which you NEED for daily use.
I will be trying the beta software, but I’ll put it on a device that I don’t use all the time, like my iPod Touch for iOS 9 or a separate hard drive for OS X 10.11. I will NOT install it on my iPhone, iPad or the main partition of my Mac.
One nice thing is when iOS 9 and OS X 10.11 come out later this year, if you are running the current versions (iOS 8 or OS X 10.10) you’ll be able to upgrade, and you should upgrade so that you have the most current security updates.
Of course before you upgrade, make sure you have a good backup.
For iOS you can backup either by connecting to your computer and syncing with iTunes. You can also backup with iCloud without connecting to a computer.
For OS X, I suggest making a bootable backup with a program like Carbon Copy Cloner (which I use and find very easy) or Super Duper. You can also make backups with Time Machine which is built in to OS X.