[bombledmonk]

I've spent 9 years maintaining a userscript for a website that now has a new beta in React. Boy, is that a pain in the behind comparatively. I'm pretty sure what I'm doing on the old version of the site won't be possible, but that is yet to be determined.

[jjordan]

You may want to check out Social Fixer (socialfixer.com). It's a userscript that manipulates and adds features to Facebook, which is written in React. You may get some ideas from them.