I grabbed the first RC from Jenkins very soon after it was built. Living dangerously, you betcha...
So far, so (mostly) good. The Go To Airport dialog works, yay! If/when METAR loads, the clouds look better than ever, dare I say terrific, and now actually extend way into the distance, if that's how you set them. I say if/when, because often it needs a nudge from the apply button in the weather dialog.
However, the red glideslope boxes still dive underground well short of the runway, just as in 2.12. Also, navdata often wants to reload at spawn, which takes several minutes (5-10?), and if terrasync is enabled, it goes into an endless download even if you already have the scenery, not spawning your plane until scenery download is disabled from the menu. That's a pretty big bug, unless some new undocumented feature is going on that I don't understand.
I don't see the shallow water color around tropical islands, though enclosed lagoons are turquoise now. Maybe that's another Mac driver issue, like Rembrandt. Frame rates don't seem different from 2.12. It was late, and I only saw two other planes (777s) on the ground, but both were at least half submerged in the tarmac. I was not.
Shift-escape resets are still buggy. If I spawn onto AI Nimitz and do a reset, my plane tries to respawn above the middle of KSFO, typically crashing when it buries its nose into the grass. 2.12 did that, too.
Hope this helps. I'm sure I'll find more stuff, both good and not so much, over the next few days.