i'll preface this by saying that i might be biased. when i was young and had a Nintendo DS, the only game i played as much as New Super Mario Bros and Mario Kart DS was Geometry Wars: Galaxies. beyond that, i would play every freeware twin stick shooter that i could get my hands on. on top of that, i played Playstation Network titles like Super Stardust Delta and Galaga Legions DX (which is a twin-stick shooter btw! i feel like no one knows about that game.)
as tasogare66 also mentions, it's much more Robotron/Smash TV/Geometry Wars/Waves than something like an arcade shmup (STG). i don't only mean this in the sense that you can shoot in 360 degrees of motion -- you are carving out your own space by mowing down waves of low-health enemies, and paying attention to where they spawn, rather than dodging bullets. in fact, i think all lethal objects in the game you can mow down with your gun, including the projectiles fired by the boss. it shows you do not need bullet-hell danmaku-style attack patterns to make a game very challenging and rewarding! that isn't to say games that rely heavily on complex bullet patterns are bad, of course. but i think games that rely on enemy swarms are harder to design for, if only because there are fewer good examples. the conventions for the genre are less well established. which is part of why i'm so impressed by the design of this game. the way the waves spawn in, the enemy types that work together to create complex and challenging situations... it's so good.
the smash attack is awesome. you create an area-of-effect when you release right click, as well as when you ricochet off walls. it's clear that the wave patterns are designed so you can take out dozens of enemies at once if you time it right (and it feels so good when you do!) it's also a good defensive option when you feel pressured. i'm still getting better at it using it creatively
i believe the boss is always the same, but if you live long enough you can see it multiple times. unfortunately in the web version, the game crashes if you live too long due to the replay system. but if you play the downloadable version it's fine!
edit: looks like the developer has fixed the crash on the web version
edit: here's some gameplay