Indeed, nifty. I've been using this (usually with Google rather than DeepL) for some time with French. I type my French, switch to English, then back. It usually gets the spelling and grammar better than I could.

Of course I could just type it in English in the first place and let it auto-translate. This is just for practice. And I always check to ensure that the result is what I wanted (as best I can).

It would be nice if they had a tool for that directly: pointing out the mistakes, keeping properties of the original text (like formal vs informal you), maybe even a grammar lesson to explain what I got wrong.