Excellent!
I honestly don't see why people are complaining about this game. The only problem was the lag.
The difficulty was perfect, and the enemies got stronger on a very good curve. The maps were well laid out, and each had it's own challenges. The length of the game seems excellent as well, will definetly put some playing time in until I beat all 10 levels.
I played through Bridge Alert until I had to quit out. The only problem is the repetitiveness, the same strategy seems to work for each different level.
And people, fighting the planes REALLY isn't difficult. You just need to plan ahead. The planes are the fourth mini-wave in each color wave (if you know what I'm talking about), so just have all your flaks upgraded by then, and don't be upgrading any guns during that wave.
Excellent game. 5/5, 10/10. Kept me playing for hours. Hopefully the next one will have a bit less lag.
P.S. A downloadable version would be epic.