How does the app behave in Windows 10 mobile
From the version 188.8.131.52 , the app is “Optimized” for Windows 10. This means that most of bugs and crashes are fixed and the app should work flawlessly, as in WP 8.1.
The app is not a Universal app, yet. Not every Windows 10 feature is available yet, and won't be till the app will be completely rewritten.