God, I'm glad I beat it.
What's most disappointing is that, despite the graphical improvements to the previous entry, it's really the same game. Just play until the highest technology, build your turrets to the max, let them make you money for the most part, maybe throwing a medium level sentry out, then when you have enough for 3 or so of the ultimate units, throw them out there and win.
Yes, there are other ways to beat the game, but that's the most immediately apparent way, and given the way the units are set up, why bother with another way?
The addition of leveling up the damage and armor of the units may be even more damaging. It's an artificial way to make the game seem 'longer' when it's actually just bloated. It's the exact thing the-exp was commenting on when he made 'Upgrade Complete!' and also the same basic point jmtb02 was making with 'Achievement Unlocked!'. It doesn't add anything to the game -- not it's Mechanics, Story, nothing.
I voted it a 3, but I really should have given it a 2. (That would correspond to a 4 on a 10-point system, before anyone asks.)