I have been a major user of CI back from CI 2.X to CI 3.X
all my old and new projects are written on CI until now.
the easiest upgrades i had/have encountered without breaking you app.
i have seen the development of CI4 from alpha,beta to RC but this is really the first time i will be using it.
I cannot imagine the effort and dedication poured into this project to make a successful release.