Currently seems to be very poorly balanced early on (not sure if it improves later). It's impossible to get high enough damage output to last more than a couple of minutes, and by then the enemies are already notably faster moving than you can get, even if you take all of the upgrades. In half an hour, I'm half way to having enough for one of the cheapest upgrades...
Maybe once you get most of them the game becomes playable? But at the rate I'm able to gain blue crystals right now, getting the first damage increase would take 3 hours of repetitive boredom.

I'll try to keep going and update if it does improve, but at the moment I would strongly recommend against giving it a go yourself.

Currently seems to be very poorly balanced early on (not sure if it improves later). It's impossible to get high enough damage output to last more than a couple of minutes, and by then the enemies are already notably faster moving than you can get, even if you take all of the upgrades. In half an hour, I'm half way to having enough for one of the cheapest upgrades... Maybe once you get most of them the game becomes playable? But at the rate I'm able to gain blue crystals right now, getting the first damage increase would take 3 hours of repetitive boredom. I'll try to keep going and update if it does improve, but at the moment I would strongly recommend against giving it a go yourself.