I assume you want any feedback on issues / teething problems?
There seems to be at least one problem with the new count number in tabs - I get a (Bad value!) rather than a count...
And you can't select how you want a photo attached - it just attaches as full screen. And if you add one by mistake, you don't seem to be able to remove it as "Clear attachment" doesn't seem to work.
I'm using IE 11 on Windows 10.
[EDIT] Issue also on Chrome and Firefox on Windows 10.