I use Grid more than Flex at this point, but they both server very important roles.
When I want to work "Layout in" i.e. Define how my layout should work and flow my content into it, I use Grid. In my current life, I do this methodology a lot.
When my content needs to control the size of my elements, i.e. in a navigation or something similar, I use Flex and it works really well.
I also really enjoy using flex inside a grid item to control awkward whitespace issues (the hardest part of web design, figuring out what to do with awkward whitespace!).
I think the important thing when choosing a layout mechanism is this: If you have to drastically modify your markup to make the layout mechanism work, you may be using the wrong tool for the job.
I wrote an article on this a little while ago
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