Amazing once you figure it out!
I was frustrated for the longest time trying to work with the flying types, but I eventually figured out a combination of aliens that worked pretty well.
*SPOILERS* My only gripes with this game involve the alien lord, the monstrous blockade, and the power-ups. While it's handy that the alien lord follows close units and retreats to others when the attacking force fails, it gets really annoying when it finds a trap and decides to just sit there for the remainder of the game. I have no idea if this can be fixed, but I thought I'd let you know anyways. The monstrous blockade...well, I can't seem to find a use for it. All it does is sit there and absorb bullets. Perhaps it's meant to be used as a shield??...I honestly have no idea, lol!. As for the power-ups, occasionally the arrow is hidden behind the options at the top of the screen. Sometimes the arrow doesn't appear at all!
I had a lot of fun with this game though, so these minor issues won't change my rating. Keep throwing these games out there, they're really good!