Over the past few weeks I've been evaluating Julia as a potential R replacement. IMO, it's definitely not there yet. I've run into bugs, crashes, hangs, sluggish performance, packages that won't load in 1.0, a clunky REPL, and a lot of code that I can't trust has been as well peer reviewed as what's in Base R. It'll take me a couple months but I plan to write a blog post on my experience, and I'm no Dan Luu ;)
That said I like the design of Julia, and the mission or promise as I call it could eventually make Julia an R replacement (as well as a replacement for lots of other languages). I started using R in 2001-ish, and at the time I remember thinking no way would I switch to R from SAS. Today I laugh at that, but that's the difference that 15 years will make. Julia is still very young. It's advantage is the modern design. R is entrenched and robust, but let's face it, it's design is ancient, rooted in the Fortran-and-card-punch era, and there are things (eg. multithreaded) it will never do.
Julia is at a crossroads: the core developers are eager to listen to the community. Whether the devs can maintain grasp of their vision while still delivering what the community wants is yet to be seen. They're being pulled in a thousand different directions, based on everyone's desire for what they want out of Julia. This could end up a disaster if the devs don't stick to their vision. Even though it's not good enough to replace R for my daily work right now, I plan to keep using Julia and contributing, because in 5, 10, 15 years, perhaps I will feel about R the way I feel about SAS today.