A good game, but a few steps short of perfection.
I truly enjoyed the game, it was a good challenge, and fairly exciting to be quite honest, when massive swarms of bullets are flying at you, while you've got low energy. The game takes a fair amount of reflexes, and alot of quick glances at where the closest bullets are heading towards. However, it wasn't perfect, despite it's very good gameplay, in my eyes. There are a couple things I'd suggest, if you make a sequel, that would make it much more fun.
1. Some form of defense, beyond your shield. The absorb/counterattack theme was very fun, but when you reach level 17+, you tend to run out of energy too quickly, from shield sapping shots, and just an overflow of bullets. I'd suggest maybe adding an upgrade that perhaps gives you a pair of auto-firing turrets, once you've aquired a specific amount of upgrades. Nothing TOO powerful, but something that can at least give you at least a little less stress when it comes to how many enemies are swarming after you. Having them pick off a few for you, would help there.
2. Alternate shield modes. This might go against the simplicity of the absorb/counter theme, but perhaps once you've worked through enough upgrades, you could gain a new style of shield? Maybe one that just bounces enemy bullets off you, instead of absorbing like the standard shield. Perhaps make it cost a bit more energy per bullet, but it would be a nice change of pace, from the standard charge, fire, charge, fire, shield. Or maybe a shield that creates spinning orbs around you, as you absorb more, that could damage enemies they touch? I'm certain there is a variety of other shields that could make this game even better, if you brought them in.
3. Some sort of super-charge weapon. To put it simply, maybe something that charges slowly over time, as you absorb and fire back bullets, that you can pull out as a trump card, in case you're being overwhelmed by enemy fire. Could be something as simple as a bomb that destroys all nearby enemies, or maybe even some temporary homing missle launcher, that blasts near by enemies for a short duration. You could even make it a multi-level super weapon, that gets stronger the longer you charge it, or possibly could be upgraded for a nice chunk of Tech points. It would spice things up, in my eyes.
Those suggestions aside, I really did have fun playing your game. It had me sitting here for well over an hour and a half, blasting back bullets, and watching myself get destroyed once the screen was full of enemies, yet I enjoyed every moment of it, even when I did die. Add a couple more features to your next game, if you make a sequel, and I'm certain it will be a hit my friend. Best of luck in the future!
-Cossack.