From http://www.drugpolicy.org/marijuana/medical/challenges/litigators/medical/conditions/epilepsy.cfm
Clinical experience and emerging research further indicate that marijuana can help control epileptic seizures.(45) Cannabidiol (CBD), one of the primary (and nonpsychoactive) cannabinoids present in the cannabis plant, appears to be of particular benefit, allowing patients who ingest it at certain times to avoid seizure activity. Some epileptics who cannot tolerate other antiseizure medications have been able to use marijuana to successfully control their seizures, without experiencing debilitating side effects.
From http://www.mellowgold.com/medical/epilepsy.html
Some people with grand mal seizures say they can prevent their seizures entirely by smoking marijuana. Others, who suffer complex partial seizures, report that marijuana also curbs their symptoms and prevents loss of consciousness. Marijuana is not considered useful for treating petit mal or absence seizures and may even worsen them.
Some patients find that marijuana works in conjunction with other drugs they are taking. Others find that marijuana works best for them when it is used without other drugs. Either way, these epileptic patients have made marijuana a necessary part of their medical treatment.
People using marijuana to control epilepsy should be aware that withdrawal from any medication that controls seizures may leave you more susceptible to the seizures. Marijuana is no exception. Patients with epilepsy are advised to exercise caution when using oral THC because there is not sufficient knowledge about the convulsive or anti-convulsive properties of the single compound.
Other websites of interest:
http://www.rxmarihuana.com/epilepsy.htm