I'm in the beta and I have played Horizon. I'm not impressed at all. The worlds are tiny and uninteresting. The color pallet is cartoony. Audio has enough delay still to be unnatural (a problem with cell phones too, btw, but made much worse when the person is standing in front of you).

Their world building tools are kind of interesting, and surely were difficult to implement, but lack any real purpose or traction.

All in all, I'm surprised more by how basic and bad Horizon is than I am by how good it is. Its been in development for a long time, and by now, I expected a lot more.