The main gimmick of the game is each stage is a generation of gaming in space shooters, so the game has an evolution theme. Level 1 is like Space Invaders, 2 is NES, 3 SNES, and 4 PS 1
(then a final stage).
Your ship and enemy ships have what you'd expect from that generation, but the whole thing is on steroids; many more bullets than you'd see normally for a game of that time, with pumped up graphics, and smooth controls right from the get-go. In other words, even from the start it's a lot of fun.
The later levels are longer, and have tons of bullets flying around, even on the Medium setting; good thing your hit box is nice and small by that point, and you can even deflect bullets by grazing them in the last level. It's really just the kind of game where you're always thinking. On the more standard difficulties, you get lots of lives, and earn more through the game, so it's usually not mega frustrating to take a death here and there.
The other big gimmick (also relating to the evolution theme) is that meeting certain objectives changes or adds a weapon to your ship for the rest of that playthrough. After beating the game once, it'll tell you what you need to do for each weapon, though some are hidden behind others, and most of the objectives are pretty hard.
It really makes you want to play more and uncover the secrets of the game. I haven't played it enough yet, but I guess on the harder difficulties it's even harder to meet those objectives, and more important to get the resulting weapons. You're also ranked by your score and can save your replays (which you can watch in super-fast-forward), so those help encourage a good run with good weapons.
The music is a sugary delight. The samples are just okay, but the way they're arranged is excellent and goes really well with the bullet hell action especially.
The game is in Japanese but there is English in smaller lettering for everything. The translations are somewhat rough but still very clear and convenient.
If you have a gamepad for your PC, downloading Joy to Key (web search it) is essential for this game.
Overall it's simply a great game and I'm glad I came across it. For freeware, easily four stars out of four. If it was $10, I'd give it three-and-a-half stars out of four.