1. Most people consider it completely safe in reasonable quantities, with cycling.
2. Creatine supplementation definitely improves athletic performance and muscle growth in people who have a creatine shortage for some reason, either due to disease or due to creatine-poor diets, such as vegetarians.
3. Creatine definitely increases the size of muscles due to muscle tissue water retention; most people who take creatine see this effect and believe creatine has "worked" because of it. Of course for most people, merely looking strong is the goal.
4. Modern diets rich in red meat provide dietary creatine far in excess of what the body needs. Even without creatine supplementation we have far more than ancient man.
5. There is no scientific evidence that creatine supplementation in healthy people actually increases athletic performance, though this is widely accepted in the weight lifting community. (note that lack of evidence here doesn't mean it's not true, since sport performance studies are uniformly poor).
6. Cyclists and other weight-sensitive athletes actually think creatine is contra-indicated because the weight gain due to water retention hurts performance.