I was just recently trying eM Client, was happy at the beginning as it has nice interface and integrates with GMail contacts and calendar. Was considering even buying the license. But then I started having contact local/remote sync conflicts without having them changed on either side. And then while the modal was blocking UI, it apparently was blocking the scheduler, so it was queuing all mailbox sync operations and performs ALL of them multiple times after modal is closed...

So I guess back to Opera Mail. Other ones like TB, Sylpheed don't cut it for me.. I guess they have UI and network actions within the same thread, and that makes them get stuck.