It is difficult, but for me it hit the "hard but enjoyable" spot just right. It was a fun challenge where every time I played it I got a little farther, so it was rewarding. I think I spent around 25 minutes playing it.
One thing that was a bit frustrating was the first alien. You have to hit them with either the first or second stone, and there is a hill in the way that makes it pretty hard. So if this first guy was a little slower, or the hill was not so much in the way, first time players would probably find it a little bit more enjoyable.