But got pretty boring around round 20. Just kept using an artillery strike at the point where the enemies enter the field and it took out most of the enemy artilery making the rest of the enemies tank fodder. I prefered it over the later support because it was a long lasting barrage of devastating fire that killed everything but air units and was immensely cheaper as well.
That being said. It is fun, but if gets old quick. Maybe there's only 30 rounds and I just didn't make it that far or something (made it to 26 or so), but it seems like there needs to be more of a challenge after a while. It just became to easy to be fun any longer.
And regarding upgrades, i don't like the transport helicopters at all. I found it too annoying to try to select them in mid-air and tell them to drop my units where I want them. If it were a support-like option it would have been handier. You know, clicking the transports in the menu and then where you want them to drop troops/tanks. That or a ralley point for all transport choppers so you set it and they automatically drop there.
I did notice, however, that the transports were wonderful for an extra 5,000 cash. It cost 20k to build one and if it flew threw the end of the screen it gave me 25k refund. Since they were ariel and fast and all my enemies were generally preoccupied with my other units by the point I could purchase transports, I effectively built them nonstop for the sole purpose of extra income.
My biggest recommendation would be to add more and more troops every round. Maybe they already do increase (even in later rounds) but I didn't notice any difference after a certain point. It just seemed like I was killing the same wave over and over. Maybe I'm just really awesome and found about the best possible defense so that nothing could make it through and simply caused me to not notice the increased difficulty. But I doubt it. Fun, but gets old relatively quick. I feel like my only limitation was my interest. At least if it got really difficult and was neverending I could feel satisfied when I died off at round 45 or whatever. The conclusion... I just feel a bit unsatisfied that I either don't eventually lose or achieve some sort of ending goal, like a set number of waves.