Today, Apple kicked off WWDC 2017, where it announced the next major updates for iOS, macOS, and watchOS, with tvOS news coming later this week. As usual, all of the Developer Beta builds are available now, including tvOS 11, which hasn't even been formally announced.
As with all first betas, you should proceed with caution here. There's a reason that Apple hasn't seeded this to the Public Beta Program yet, although those should be coming later on this month for iOS and macOS.
For example, the iOS 11 release notes show a number of known issues to be aware of. In fact, many third-party apps are cited as possibly crashing, including DirecTV, Kindle, MobileIron Mobile@Work, Facebook Messenger (when Facebook isn't installed), Amazon Video (playback controls might not work), Pinterest, Tencent Video, and Skype. Also, migrating from an Android phone might not succeed. There are dozens of other known issues.
When upgrading to macOS 10.13 High Sierra, you'll be prompted to convert to Apple File System, but HDD-only Macs won't do it by default. Some Macs aren't supported yet, such as iMacs with 3TB Fusion drives and BootCamp, and the mid-2012 Mac Pro.
But those aren't the only four betas that Apple seeded today. Xcode 9 beta is available, which should support the new APIs that are included in the new operating systems. There's also Swift Playgrounds 2, macOS Server 5.4, Classroom 2.1, and Apple Configurator 2.5.