IOCP is the sensible way to do it. Unix, for a system where supposedly everything is a file, has a lot of very specific behaviors for "special" kinds of files. In contrast IOCP lets you treat all async operations from timers to sockets to disk the same way and the thread pool does a decent job of scaling too.
I've used both programming models, and I find "can I read/write this fd" much more intuitive and versatile than "try to do it and let me know when actually done". In particular, you can use many different models to distribute and parallelize work with poll/epoll.
Also, poll/epoll get even more versatile with current Linux systems, which take "everything is a file" much further with signalfd, timerfd, and eventfd.
The problem with IOCP is that it is more memory intensive as all memory for outstanding operations must be pre-allocated. With the readiness model, you can use pools of memory instead for dramatically less overall memory usage. There is a hack to use 1B reads with IOCP to get around this, but it doesn't feel very clean.
