Don’t Install if You Use a Really Old Mac
Are you using an old Mac? Like one of the last models to be compatible with macOS High Sierra?
If you answered yes, we recommend waiting to see how this experience is for other users. There is a chance the update could run great, but it could also be slow and buggy on an older Mac.
There are also times where some features don't come to the older devices so you may not get all of the benefits of upgrading anyway. For example, you won't get the new file system if you aren't on a SSD.
We recommend checking forums, Apple Discussion Boards and twitter to find out how your specific model is working on macOS High Sierra -- before you upgrade.