I especially like them for convention play. It's likely that someone at the table hasn't played the game. A cheat sheet helps them to be involved faster.
A good cheat sheet can't replace a rule book or a GM's requirement to know the rules. At the most it should be a one page document with the core rules and perhaps page numbers for obscure rules. That lets the players know that there are rules for other things but that they won't need them all the time.
If there isn't one available for a game that in running at a con, then I always make one for players. They need it. The limited time slot needs it. I want to maximise the time spent telling the story and minimise the time spent checking rules.
After all, it's supposed to be about the story, right?
