Really good feel to this game. I loved the palette changes between levels and the banter interstitials. I did notice a bit of smooth sub-pixel movement, which may just be an engine limitation, but I'd recommend trying to turn this off next time to really be authentic GameBoy.
I always loved these games as a kid, I played one called OverKill by Epic a lot. I remember my Dad went and bought a joystick for us especially to play. Thank you for bringing back that memory!