Many POCO users report using it alongside Boost, so it is obvious that there are incentives for people in both projects. Boost is a collection of high-quality libraries. But it is not a framework. As for ACE, I have used it in the past and did not like the design. Additionally, its support for ancient non-compliant compilers has shaped the code base in an ugly way.
What really distinguishes POCO is a design that scales and an interface with rich library availability reminiscent of those one gets with Java or C#. At this time, the most acutely lacking thing from POCO is asynchronous IO.
