I agree, flexbox is refreshing. Couple it with autoprefixer or a flexbox scss mixin, layout a design with it for the first time and you won't go back.

For people that are use to hacks + grid systems, it seems weird at first, but over time you will become familiar with a rich layout system and stop thinking left-to-right, it will become more natural. The language of the spec and the declarations (align/justify distinctions, etc) can be weird at first, but you get used to it.