I'm a fan of the user space threading for IO programs that handle more than one stream. It gives you the same context you are used to in blocking programs without the mostly needless thread context switching.

My context is that most of my recent years work was in high performance systems that handle thousands of streams concurrently and only have a limited amount of cores and a rather limited amount of processing for each io request have so the context switching cost becomes a high percentage compared to actual work done.