Probably want to stick to 4.2.1 GPL [1] (there are GPL versions on 5.x and 6.x but only minor point releases)
I think it's great for enterprise form-fill-in applications. It's a complete widget set for doing in the browser what GUI apps from 20 years ago had in Java (for example)
I have no interest whatsoever in writing web applications by using multiple components from multiple sources. I want to use a single toolkit. ExtJS satisfies my requirements. The tooling is awkward, but I've learned how to use it.
As a counter-example look at Sencha, who have long alienated developers using their ExtJS toolkit, and this has deepened since the takeover by Idera.