The efficiency of keeping everythign so well organised. To second that it is so easy to quickly adjsut templates for campaigns and flows that need to tweaking and refining.

The most amazing thing is that you don't need to know what's inside, just design your email and that's it.

I have tried multiple such tools, but Designmodo Startup is on the top of my list. First is the simple and straightforward drag and drop system. Components/Blocks help you visualise what it should look like and how it will look like. And are very easy to customize. Second is the amazing presentation of the tool. The UX is amazing and kudos to the person who is behind it all.