Oh, I just found this bit out. Why are there so many 5 star reviews? Well, the game prompts you to rate it and says "Give us 5 stars to receive free gems!" Interestingly, the developers can't actually tell what rating you gave so you could give it a 1 star rating and still get the gems, but the in-game prompt of course doesn't tell you that.
The good news about seeing this is I had a quick look around and I found my CD copy of Dungeon Keeper. Gonna see about getting it going.