Geometry Wars is just straight up good. It's pretty beautiful to look at and it's fun to play. I love the sound and music.
It's very arcade-y in nature, for sure. Having said that, the gameplay is also predictable: although you'll get different types of waves, you get very similar ones at similar times. It's balanced and eases you into gameplay, but the start is also fairly boring after you've played a few times. Conversely, later in your run things will get ridiculously hectic.
This was the first game I had played where I had a lot of issues with my controller(s). I've had similar issues elsewhere now, but Geometry wars (when I started it a few years ago) was only really programmed for the 360 controller. Other controllers would sort of work, but the game didn't really understand the input so much (for example, the game couldn't detect "Up" on my ps2 controller).
While this is one of my favorite chill out games, I'll probably only play it for about 15 minutes at a time and then not touch it for a few months. It's great fun for sure, but it's also a very redundant game.
If you don't already have it, though, you absolutely need Geometry Wars in your library.