Basically, you need to pay a membership fee in order to be able to enter these clubs, but there are added benefits as well; You're able to carry a large quanity on you at all times, and you're also allowed to grow a small portion of your own plants if you so desire.  It's under Proposition 215.

Medical marijuana patients are still susceptible to Federal prosecution, as on a Federal level, it's still 100% illegal.

Some info:  

http://www.usdoj.gov/dea/ongoing/calimarijuana.html

http://www.chrisconrad.com/expert.witness/Prop215.html

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The text of the Prop 215 initiative follows:
Section 1. Section 11362.5 is added to the California Health and Safety Code, to read:

11362.5. (a) This section shall be known and may be cited as the Compassionate Use Act of 1996.  
(b) (1) The people of the State of California hereby find and declare that the purposes of the Compassionate Use Act of 1996 are as follows:

(A) To ensure that seriously ill Californians have the right to obtain and use marijuana for medical purposes where that medical use is deemed appropriate and has been recommended by a physician who has determined that the persons health would benefit from the use of marijuana in the treatment of cancer, anorexia, AIDS, chronic pain, spasticity, glaucoma, arthritis, migraine or any other illness for which marijuana provides relief.  

(B) To ensure that patients and their primary caregivers who obtain and use marijuana for medical purposes upon the recommendation of a physician are not subject to criminal prosecution or sanction.

(C) To encourage the federal and state governments to implement a plan for the safe and affordable distribution of marijuana to all patients in medical need of marijuana.

(2) Nothing in this act shall be construed to supersede legislation prohibiting persons from engaging in conduct that endangers others, nor to condone the diversion of marijuana for non-medical purposes.

(c) Notwithstanding any other provision of law, no physician in this state shall be punished, or denied any right or privilege, for having recommended marijuana to a patient for medical purposes.

(d) Section 11357, relating to the possession of marijuana, and Section 11358, relating to the cultivation of marijuana, shall not apply to a patient, or to a patient's primary caregiver, who possesses or cultivates marijuana for the personal medical purposes of the patient upon the written or oral recommendation or approval of a physician.

(e) For the purposes of this section, Primary caregiver means the individual designated by the person exempted under this act who has consistently assumed responsibility for the housing, health or safety of that person.

Sec. 2. If any provision of this measure or the application thereof to any person or circumstance is held invalid, that invalidity shall not affect other provisions or applications of the measure which can be given effect without the invalid provision or application, and to this end the provisions of this measure are severable.

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