I like this game but there are some things I want to tell.
Well first score system is not obvious for somebody new. Maybe Metal Slug had similar but I was surprised when while playing 2 time I saved more kids and get lower score. On my firs run I didn't save any because I was struggling with boss. On second run I had saved 6 of them and still got lower in top players. Apparently there is combo based system. It is a small thing but I think you should be open what is giving more score.
Second one is that when teaching control game is like you can do this this and this. Go. No tutorial or even try to do it yourself. I guess it is for imitating arcade games but arcade controls are displayed while showing movements to player that will play it on keyboard or game-pad.
Third one is that you need to press coin button. Again I guess is for imitating arcades but I don't understand why do it on PC.
This is 3 things I think could be better or worse, but overall I would buy this game today if there was a full version and if it was priced adequately.
Control is not complicated so you can pick it on the fly, score system you can learn by observing what the game is doing and pressing the coin button is not a serious issue if you played any arcade games on MAME.