Man I'm so addicted to this. I have all the achievements now except the 50,000,000 pts and some of the 10/25 level completions. The game is really full-featured and a lot of fun. Especially the VR levels which can keep things interesting for a long time.
The key to success in this game is placement of slow towers and management of "wasted" rocket fire. What happens with fast units is once they get past your defenses they draw much more firepower than is required to kill them because the rockets take too long to catch up. So once the first in a string of planes get past your anti-air missiles they all break through. At higher levels you have to put as many anti-air missile towers as you can on the flight path intersections and in the middle of the board where none of their range is wasted. Whether you pass a level with no losses is often times a matter of luck since the units that come out are randomized. If you get a rapid stream of bombers and then a rapid stream of jets a few seconds later you may be screwed even with ideal defenses.
Unlike some other tower defense games the ground units are pretty easy to handle. There's no need to learn advanced techniques like juggling, just finding good layouts is all that's necessary.
Man I gotta finish all the achievements so I can retire