Like many people, started playing this back in the early Xbox 360 days, and I find it still holds up as being an extremely enjoyable arcade game, and if you at all enjoy these types of games, I really can't recommend this enough. Geometry Wars feels like an homage or love letter to classic games like Defender, Space Invaders, and Asteroid.
The gameplay is extremely fun and finds the right balance of casual gameplay and simultaneously being very challenging while even approaching unforgiving/punishing in how hard it can be. It's hard to explain the experience of how playing this game can be relaxing and leasurely and, in the same session, become intense and anxiety-provoking, causing a release of adreniline in the player. It's also difficult to explain how rewarding it is to feel yourself naturally responding to the challenges the game throws at you and finding yourself handling situations and reacting differently, getting through a point in the game that used to have you stuck. The game itself is very well made and has a great feel to it, and the graphics are beautiful.
My only two minor quibbles with the Steam release are lack of an online Leaderboard (probably has something to do with Activision getting shutdown and that they just used Xbox Live's leaderboard functionality before) and that the Steam release doesn't have any of the unlockable achievements the Xbox 360 version had (how hard would it have been to include those?).
I payed $1.99 for this on Steam and would have actually paid more. Again, I can't recommend it enough for a modern take on classic Arcade gameplay.