Have been using Vivaldi for a few months now. It's very fast and it has never crashed on me the last 3 beta releases. I really enjoy the UI color following the color scheme of the visited site!
F11/Fullscreen - from here I cannot seem to reach the menu or tabs
Save Open Tabs as Session - This includes all tabs from all windows in a session. I'd rather see this implemented as saving all tabs in a single window.