I really liked it,...
I really liked it. It's definitively a unique "tower defense" game, but at the same time very different. In particular I like how you use fewer towers than you'd typically use in a traditional tower defense game. I would say that this is more of a "smart tower defense game" because it requires you to think more.
What I really didn't like was the random colors. Clearly if you waited long enough you could get any color combination. The only "limitation" is you can't have all the same color. What I would prefer is to ditch the random colors all together and just let the player use any upgrade they want, as long as they don't fill each upgrade circle with all the same color (Example: 2/2 blues, 3/3 blues). To me the game was hard enough without random upgrade slots... It's more annoying than anything else.
I also think the "Command Shell" is quite usless until level 12. Would be better if it had 3 upgrade slots instead of two (All the upgrades you want for it are red). The "Compressor" is useful for early parts of the game, but later on it's just too weak.
I found a good combination on quite a few levels was the "Sentry Tower" + "Linker" + "Optimizer" + "Range". Two of these in close proximity covering the entire map is deadly (Not on level 12, or any with a lot of enemies that reproduce). Sometimes I like to substitute "Range" for "Fork".