I have adopted ever since it's introduced at WWDC back in 2014. Objective-C is very powerful and very matured language. Language constructs are not something familiar to rest of the programming language community and there are too much of C.
Swift 1.0 had so many rough edges and it did not have several things like exception handling etc. like Objective-C. The compilation was very slow due to the advanced optimization. All of sudden the MacBook Pros started looking very slow with 1.0.
Surprisingly Apple quickly iterated the language and there were serious breaking changes between the minor versions and major versions. Certainly, projects went slight budget overruns, very hard to find developers who are familiar with Swift.
The most interesting thing is that, we created more iOS developers and the grads joined in my company out of the university quickly picked up Swift much better and faster than Objective-C. We are still struggling to find developers with Swift experience though the language is in the 4th and most solid versions. Many ocmpanies are still considering to upgrade to Swift but not there yet.
Despite of everything else, you write half of the code than Objective-C. The language constructs are modern and you are faster as you write a scripting language. Optionals extensions and enumerations are super powerful and give new meaning in the world of Swift. It works beautifully across existing Objective-C frameworks and code. There several quirks and lot may things to improve but it's certainly important to learn Swift and it's going to be the future. The community is super active and the language is getting improved at godly speed. Also the Xcode assistant to migrate the code between the version is very helpful.