The game is very smooth, and very appealing in terms of the visuals and especially the audio!
One thing that annoyed me was the progression curve. It never felt like the asteroids/UFO's hitting the earth were my fault for playing poorly, rather that my damage output never got high enough to handle them even buying upgrades wherever I could. Like, it did not feel like it was physically possible to actually clear most waves without taking hits, no matter how well I played.