With the new compose enabled, Gmail no longer takes you to a different screen to write a message. When you click "compose," a small message box pops up in the bottom-right-hand corner of the page. From there, you retain all the functionality of the old compose method - it's just smaller and looks more like a chat box.
"You are busy people, so it's no surprise that an overwhelming number of you opted to try out Gmail's faster, simpler compose experience after it launched last October. In addition to telling us what you love about the new compose experience (like how much easier it is to multitask!) you’ve also been sending us helpful suggestions for what features you'd like to see added. As a result of your input, we're now ready to introduce the new compose experience as the default for everyone. We're looking forward to hearing what you think!" says Gmail Product Manager Phil Sharp.
Over the past few months, Google has been tweaking the new compose feature and has added the ability to send files with Google Drive, pop-out replies, and starring and labeling while composing.
Google says that the rollout will take a few days.
Overall, the compose box is pretty useful in some circumstances - especially when you're writing an email and you have to reference another email. It simply saves a lot of time. You can also write multiple messages at a time and minimize any of them to finish later. It's a solid improvement to the Gmail experience, and I doubt we'll hear any bitching from Gmail users as the rollout proceeds.