Side-scrolling shoot 'em up Graceful Explosion Machine [official site], out this week, has one of the better combat systems I've seen in the genre. Your spaceship has four weapons to cut down the waves of blobby enemies: a simple blaster that can overheat and three flashier weapons (including a devastating missile launcher) that use up energy, which you collect from dead enemies. So, you've got to balance aggression with conserving your resources, and it's a neat internal tug-of-war.
It's pretty hectic, too, with lots of enemies on screen at one time, and in its latter stages is pretty damn hard. This is all from playing the PlayStation 4 version, by the way, but I can't imagine it'd be any different on PC (although you should definitely play with a gamepad).
Here's a trailer to give you an idea:
The levels, of which there are 36 across four themed worlds, are all infinite loops, so it doesn't matter if you head left or right. You can even stay in one spot: enemies will spawn around you and once you've killed a certain number the stage ends.
Its longevity comes in chasing high scores and competing with your friends atop the leaderboards. If that's your bag then you can buy it on Steam for a 10% discount until Tuesday at £8.99/11,69€/$11.69.
And if you're a fan of the genre, it could be worth reading this ode to shmups that Wasim wrote in 2015 using short, mechanical sentences. I just stumbled across it and really enjoyed it.