It's such a nice development on general idea of things like Powdertoy. I've got really hooked up, there's such a satisfaction of building a foundry and making it work.
Not everything is perfect of course. I would very much like to have ability to hotkey brush size increase/decrease, it's a bit of a hassle to constantly click buttons with yout mouse each time you need to change it, and I find myself changing brush size very often. 
In addition, I very much hope that dev will expand on gases. Right now overall gas system looks more like a placeholder. There's no air per se, every empty place counts as air (AFAIK), which makes theoretically interesting processes like pyrolysis of wood and coking impossible.