if you press forward and backward at the same time, the ship goes down. This is not good. Instead, you can do one of the following techniques that are mostly used: either when front or back is pressed, and then you press the other key, it's ignored, or you can go with the 'newest keypress overrides' mode. Both are equally fine. front+back=down doesn't work at all, though.
Since there's no enemy shooting at you at the moment, it's hard to tell if the gameplay has any merit. It's all down to how difficult it is, how difficult it feels and how much it makes you want to replay... quite the x-factor in this sort of game. When it's more finished there'll be more to say.