As a Californian, I will tell you that it is still a crime, federally, to use marijuana medically in California. However, because it is legal by state law, we don't use state money or resources to prosecute people who are abiding our state laws.

It doesn't stop the feds from cracking down, and anyone with a license knows that, if it came to it, they can still be arrested and charged at the federal level. Thankfully for those that need it, Obama's administration has stopped raids on medical marijuana clubs in the state, but it was a problem before then.