I've been going on a bit of a demo binge the last couple days. So far Flock is the one that I liked most, but I'm only about half way through my stack of demos. Plus I'm biased because BIRB!
My only gripe with it is that I can't change the altitude of my bird. I'd like to be able to dive bomb without tracking down a thermal vent. And I'd like flying through a ring/tunnel to be a little bit more than "approach until you see a marker appear and then press dash to activate a non-interactive animation." It would at least save me the indignity of circling around the entrance to a tunnel trying to get the damn trigger to spawn.