It's pretty good. A few clients are still PITA (looking at you, Outlook 2007 and Yahoo mail. You still have to do jump through hoops for accurate background images, or fancy things with borders.

The actual markup MJML generates is terrifying, and can be very large payloads. A simple hero graphic, some styled body text, a few buttons, and a footer was around 60KB/message. It got gnarly for us as our MTA had a max-payload size per API 'send' call (Mandrill) so our max send rates were partially constrained by MJML's fat markup.

But totally worth it to not spend _as_ much time satisfying the mail clients of the world.

I agree with all of this. When you really start to dig in to cross-client compatibility (and you observe what MJML does to provide it) you start to get a sense that NOT using a tool like MJML is a hopeless path.