I suppose this comment comes too late to have much visibility, but as for me, I can deal with the fullscreen popup thingy.
The inability to default to replying in plaintext to HTML mails is annoying, but I can deal with that, too.
What I can't deal with is the broken UX for the fake HTML elements, which were previously native controls that worked fine.
The To field is now a magic list of clickable email addresses instead of a regular textbox. Great, but clicking and dragging to select no longer works, typing with a selection active does not delete it, and undo no longer works.
The From field (for use with multiple email addresses), which used to be a native , is now... a HTML reimplementation with no additional functionality. It sort of works from the keyboard - you can press space to bring up the list, up/down to choose an item, and enter to select. But it's not quite the same (as OS X, at any rate): the list starts with no selection rather than the current item, you can't press space again to close the list, and you can't choose an item by typing. In fact, if you try to do a few things in the new compose, including typing on the list, the keystrokes pass through to the greyed-out main window and you perform random email actions.
These interactions are not inherently critical: I could easily learn to live without them in general. But they work for every other like control in the OS, and they used to work here, so naturally I have and will retain muscle memory for them; the result is infuriating.
I'm actually quite surprised nobody has made a user script to fix these and the other issues in the three weeks since my account was forced onto the new compose; Gmail's obfuscation makes this a little more annoying, but not too bad if you stick to modifying elements rather than directly interacting with the JavaScript. I'll probably end up writing one myself.