Five minutes of fun.
I actually struggled for a minute or two at the glass roof - not realising I had to shoot down. To me, it looked like some sort of portal so I thought I just had to jump into it, then thought maybe I had to press down or another button... only when I 'saw' what it was after coming back to it did it become obvious how to get past.
I feel like maybe having a sign teling us 'you can shoot downwards!' would have made that a bit more njoyable to me - it wasn't a puzzle, just me being dumb, I guess.
The first time I played, I died at the final boss, having accrued a few mistakes by that point - specially in the upwards chase and elevator scene. I completed it on my next go - glad I didn't have to go too far back but feeling that the 2 bosses seemed a bit repetitive.
The final boss specially seemed like it was really forcing us to learn the attacks and hit boxes rather than react or use much skill. And there's nothing to suggest that standing within grasping distance of the hand is a better option than trying to jump over - until we fail or realise that it never actually uses its fingers. I guess that's maybe your love of old-school game mechanics showing through.
I loved the visual signals clearly showing trails for bullets, colours distinguishing friendly fire from enemy fire.
The backgrounds were superb and I specially loved the windows just after breaking though the glass. The touch of blue lighting after the elevator and the lights by the clown, which we could smash also helped make it feel more involved.
Controls were excellent. Whilst we couldn't easily fire in a direction we weren't moving, it wasn't generally a problem and replaying it to run circles around enemies was a joy.
I found the ending - with the explosions and ensuing downward fall - lagged slightly.
Overall though, it was a real joy to play. Short, focusing on the 'five minutes of fun' and then ending before repetition ever has a chance to occur.
Variety throughout, easy to play...
This is what more flash games should be like, rather than spreading themselves to thin or competing with modern console games.