If anyone is interested in an experience report, I've been using it for about three years and it's exactly the same experience as VSCode (even works with Live Share, for example).
This means you might find that extensions you use in VSCode aren't in Codium. This is usually remedied by just downloading the extension manually yourself or by asking the extension developers to cross-publish.
The main thing you'll have to contend with is that the extension registry for VSCode is different from Codium, as Geoff says here: https://news.ycombinator.com/item?id=31606704
This means you might find that extensions you use in VSCode aren't in Codium. This is usually remedied by just downloading the extension manually yourself or by asking the extension developers to cross-publish.