Not bad, but certain limitations
I think that keeping the game map so small, you did lose out on the chance to make the game much more complex. After all, we're dealing with sprites here, which look so much better as you make them smaller. Why not have later levels with up to a 10x10 grid, instead of the 6x6, as that gives the players so many more options.
With the powerups, perhaps you could buy upgrades that improve them and on the bigger grid games, you stand more likelihood of being able to do all of the powered abilities. The four powerups that you have are great and well planned, but when the opponent gets the ability to nuke your troops, it can make the levels very hard to play, especially if they get the jump on you and your first few seconds are spent looking furiously for the first few matches to get the ball rolling.
Quite addictive and well designed, but still with room for more, I'm impressed, but I would like to see more things added to this game. A plot to keep the more-than-casual gamer occupied would also be a very nice thing. Perhaps some sort of script? Finally, I'd suggest something that allows you to turn the music off, as when you've played 20+ levels of this game, that does get very repetitive.
[Review Request Club]