Personally, I use CLOS all the time and think it's great. Even without defining my own classes, I use multiple dispatch and methods on arbitrary types quite a

Personally, I use CLOS all the time and think it's great. Even without defining my own classes, I use multiple dispatch and methods on arbitrary types quite a bit. Admittedly, I don't really use the advanced features like MOP, but I appreciate that they're there and that I can manipulate how the object system works if I ever need to. As far as performance goes, it just hasn't been a problem for me, even when doing graphics and image processing. A pattern that's worked really well is to use OOP to structure and organize everything, and then have the implementation details flushed out with functional or imperative code. It's the best of all worlds. And it's actually a good pattern in most OOP languages because it slows things down in most of them. The vtable in C++, extra GC in Java, object attributes vs. locals in Python, etc. Avoiding objects in performance critical code is just generally a good idea.