I fired up the game again today with my 7870. I must say, it runs much, much better now. Still not amazingly smooth; but smooth enough to not take me out of the immersion. And that's without the supposed performance patch that's rumored to be released soon. After that patch (assuming there even is one), I imagine this game will be running great.
Long story short: Miasmata demands a high-end graphics card, no doubt about it.
As for the game itself, it really is quite interesting. There's almost no HUD, all actions are done with your hands directly in front of you, and with your journal that you keep stored on your person. The game basically just says "You're stranded in this large swath of land. You're alone. You're sick. Find a cure to survive." And it pretty much leaves you to it. You quickly discover more about the game by reading pieces of paper scattered around; they teach you how to pick plants, research them, and how to possibly synthesize them into medicine.
A very fine game if you're into exploration. It's quite scenic and atmospheric. I'm still not far into the game, but I'm just kind of wandering off and seeing what there is to discover.