it is fun! but it also becomes repetitive after a while.
i think you can remedy the feeling of repetitiveness by adding more obstacle/enemy types or increasing the speed of the game gradually over time. these kinds of things would create a shifting landscape the player has to adapt to.
additionally, i think the feeling is impacted by the fact that jumping is the only thing that's actually difficult -- you can spam space to shoot and hold down to duck endlessly. the only thing that needs timing is the jump.
(and speaking of the jump, i feel like it could be a little higher.)
i like a lot about this game -- the style, the reactivity of the controls, the sfx -- it all fits together really well! i think some small tweaks could make it a lot more fun and a lot less repetitive :)