With the advent of Umbraco 8.1.0 it is now possible to upgrade a site from a previous version to Umbraco 8. However, this is by no means an automated process. The new upgrade feature in 8.1.0 works similarly to previous Umbraco upgrades - they upgrade content stored in the database, but don’t provide provision to upgrade custom code written in Templates, Partial Views or Controllers.
Umbraco 8 is a substantial breaking change from Umbraco 7, and so the effort needed to upgrade an existing site shouldn’t be underestimated. Critically the code used by Umbraco Developers to create Templates, Partials and Controllers undertook a substantial breaking change. Some of this change is not complex, but it will require all code to undergo changes in order for it to work with Umbraco 8.
With this in mind, it's worth considering if there is a business case to undertake the upgrade.
TL;DR For most scenarios yes. #YMMV
Umbraco 8 was launched with much fanfair in Feburary 2019, and as with most software there were some initial teething troubles which were quickly fixed. But with 6 months passing, we began to wonder was Umbraco 8 ready for use in production environments.
After much research and testing, including a practical Proof of Concept, we feel that without a doubt the core CMS is ready for use for most production scenarios. However, there are some scenarios where you should be cautious before continuing, and some where Umbraco 8 is not production ready.