I completed the game last night. My play time was around an hour and twenty minutes. I had to do a little back tracking as I missed some pick-ups required to complete the game. Overall, it's fantastic.
The controls are snappy with directional shooting mechanics that make sense. The graphic style is appealing with an almost coloured monochromatic look if that makes sense. Enemies don't get lost in the backgrounds. The difficulty level is well balanced. The always present map is clear and helpful without making the play area feel small. Any major backtracking I had to do was my fault due to forgetting to go back and collect things that were clearly marked on the map.
I am not a metroidvania fan and while this could be seen as one to a degree, it is not the type of game you are going to find yourself wandering around aimlessy looking for one little thing you've missed to enable progression.
The only issues I had with the game were that sometimes I'd not quite be in the right spot to get on a ladder resulting in getting hit by an enemy and occasionally enemies would spawn in places where they were impossible to avoid. Health and invincibility pick-ups were well placed though so these issues were somewhat negated.
Absolutely worth the few dollars they're asking for it and probably one of the least frustrating C64 games I've played recently. It still feels like a C64 game, but there's a modernity to how under control it all feels as well.