We will provide a separate extension (currently called "apcu") which contains only the user-cache part of APC. ZO+ and that extension together should behave the same way as APC currently does. This will actually work even better, because both components will be completely separate (the shared cache had fragmentation issues in APC.)

ZO+ was chosen over APC because it is more stable. ZO+ is (more or less) PHP 5.5 compatible already, whereas APC still tries to cope with 5.4 compatibility.