I was tasked with using ExtJS for a large project, without having prior experience with it. It was both a dream and a nightmare for me (OK, that's a bit dramatic, but it was a mixed bag.)

In the end, I was able to do what I needed to, albeit not on mobile due to performance issues, but I wouldn't choose to use it again if it was up to me. It just felt too bloated, and I'd much rather work with various smaller libraries, and fill in the gaps as needed.