Deserving three and a half, I gave five stars to increase its rate to what I believe it should receive. Addicting, willing to see how longly can I stand before I have to turn off the computer.
Another thing: I believe that it takes eight seconds before you can upgrade, therefore, 450 level-ups per hour. Seems like too few, huh? It's not, assuming that in about 20 minutes you get 48k.