tl;dr: Still buggy, but the feeling of exploration is definitely there already. Updates every day, the developer is serious, hard working and honest. Do not jump in right now if you expect a smooth ride since some of the mechanics are not reliable yet. One thing is bug-free: the peculiar mood, the intriguing and well-written story, the surprising landscapes, the feeling of exploration and discovery.
As this is an early access review, I want to stress the fact that the game is not finished. Not all the features have been implemented, and some implemented features do now work quite well. I'm writing this on December 24th 2014, the game has been available in Early Access for about two weeks and open for beta testing for about a month. The game has been thoroughly tested since day one. A hundred of issues have been raised; some very minor (weird pixel on button texture), some critical (crashes); and all are being addressed by the one developer, Railboy, who's been working full time and releasing patches almost every day, often several times a day.
The result is a game that is working better and better every day. From my point of view, it is already good. I keep jumping into the game with the goal of breaking it and reporting a new bug, but I end up each time walking down new paths for hours, exploring new lost villages and discovering strange events. What you can see in the trailer videos is all here already, and much more. I have put fifty hours into the game and still haven't explored half of the continent. I haven't finished the main quest either, and haven't tried 10 % of all these crafting recipes. I still haven't even cast a spell (even though I did find the dark luminite to do so)! The world is Big, in the sense that it will take you a lot of time to get to know it, and believe me, you will be lost. Even with the maps, there are places I saw once and have never found again.
Frontiers has this sandbox feel to it: you set your goal yourself and just roll with it, or you follow the story. There is no pressure to finish the main quest. Actually, pretty much everything is optional in this game; there is even an option for disabling survival if you do not want to bother killing bears for their meat (a very hazardous activity, believe me). I like the main quest very much, though, because it isn't just about big epic things, it's about people, trust, friendship, vengeance, broken hopes, and what one is ready to sacrifice in order to be the first to accomplish the impossible.
In addition to the main quest, there is something about the world itself that feels... weird. Abnormal. Out of place. It is subtle but you can tell something is wrong. I do not know yet whether it connects to the main quest, but I will definitely walk the paths of Frontiers until I find out.