I've been expecting to use Celery in a project I'm working on, could you explain why?
It is not transactional, at least not with a MongoDB back-end. If you stop a celery service, any tasks that were in progress may or may not complete, but none of them will be picked up again when you resume.
No prioritization of queues or tasks. It is possible to set up separate services that handle different queues, but that's a PITA and there's still no guaranteed order of processing.
edit: looks like they're using flask now, I don't see itty being used anywhere. They also split the web interface (resweb) into a separate project, so pyres doesn't depend on a web framework now.