1) I just turned it on, and it is not customizable at all. You cannot create custom tabs, and you can't even pick what goes into the tabs they have created.
2) When you turn it on, it disables Gmail's multiple inbox. Multiple inbox is the only thing saving my email from being a complete and utter disaster right now.
3) Judging by how they are forcing the new compose on everyone it is reasonable to expect that they will force us to use this as well. (The new compose screws up at least two workflows for me, btw. For example, sending a form email to several people is now a pain because I have to manually click "edit subject" to remove the "Fwd:"). If they force this on me, it will completely destroy my ability to use gmail. If they want me to leave gmail for good, that would be the way to guarantee it.
Now they've implemented their own version of tabs.
Just support native browser controls, please. People understand them. They're simple. They work. The more stuff they add, the heavier Gmail becomes... and the more of a pain it is to load in multiple real tabs.
"...and you can't even pick what goes into the tabs they have created."
Sure you can. Just drag an email between them. It gives you the option to permanently move emails from that sender to a specific tab. Presumably, it also adapts to your selections, much like their priority and spam filters.
The new compose has at least 4 things that are broken for me:
1) When you hit "forward", it hides the subject box, even though the majority of the time when I hit forward I want to delete the "Fwd:" from the subject line. Especially when sending a form email. This resulted in a couple of embarrassing emails before I changed back to the old compose.
2) I often want to create a big list of emails that I want to review before hitting send. The new compose replaces each email address I type with the persons name! For a lot of people I do business with, I have both their business and personal email address. But now I can't tell which one I'm about to email. What the hell Google?
3) Attachments are now an object within the email. But often I want to forward something with an attachment, but clear out the text in the email. Well now when I do select-all, it deletes the attachment also.
4) The compose email doesn't cover my screen, even when I pop it out as its own window. When I'm drafting an email, I don't want to be able to see distractions. If other emails pop up while I'm editing it, that can make me lose my train of thought. Worse, there are no options that I can set to change this.
Used to have 2 (old one, and pop out), now it has 2 (new one and pop out). (there is also the look for reply, and for chatting (which i use even if other is offline sometimes instead of a quick email), and the ability to revert to the old look)
By this logic, the old one is at least as fascist. Additionally, while you might not like the new style, "real" feature was removed, but the ability to write multiple emails at once, or write one while one is open, were added.
Edit: I just tried to old one to see it again. It is rather bulky. It needed a UI update of some kind at least. (which may have been accomplished not semi-popped out, i guess)
Technically, you still have two compose interfaces, but they have promised to remove the oldest one. To get the old interface, click the little downward pointing arrow and select "Temporarily switch back to old compose".