I am not eagerly waiting for updating my Lumia 640 XL to Windows 10. As per various reference on internet. Windows 10 Mobile is expected in December 2015. I have not tried Windows Insider options as it may hamper my day to day use. But I am very much excited to see my Phone getting updated to Windows 10.
I am already loving Cortana and some other features which Updated Windows 10 offers on desktop. My laptop upgrade was smooth and hence expecting same with my Lumia.