I used it in a personal project (<a href="http://loyaltonmuseum.org/" rel="nofollow">http://loyaltonmuseum.org/</a>), since it made things a lot simpler. Few browsers support it natively, but there's a polyfill for it (<a href="https://github.com/FremyCompany/css-grid-polyfill" rel="nofollow">https://github.com/FremyCompany/css-grid-polyfill</a>) which supposedly supports just about all modern browsers (including IE9+).
We don't use CSS Grid, 40% of our users don't support it in their browser. Judging by the five hours this post has been up, I feel like a lot of Hacker News doesn't use it either.<p>However, when support for it does go up, we're definitely migrating to it. It seems to have a really nice syntax.