Relax time = Unnecessary
The relax time is nigh-pointless, let alone unnecessary, now that I have all the upgrades. My tanks practically take care of themselves, and they're easy to replace, and they quickly regain ranks. Even so, I have craploads of money which I can use to buy more tanks or heal my tower (the latter being if everything screws up).
It's fun for a while, but only just for a while.
Good for stress relief, for instance if you imagine someone you hate sitting in the cockpit of every single airplane you blow out of the sky.
A minus, however, is lacking variety in enemies. There are only fighters and bombers, which is probably realistic (not that I would know), but I think we waved goodbye to realism once we had 6 flak tanks, 1 missile tank, two missile towers and one flak tower spamming projectiles at the enemy at a ridicule speed.
Plus two radar dishes which somehow increase the accuracy of the flak cannons.
All in all, good game for stress relief and time-wasting.
Needs more variety in the things you turn into scrap metal falling from the sky.