The most promising therapy for ALS and other neurologically based disorders at the current time is Stem-Cell based therapy, Go the Johns Hopkins ALS Clinic website and you can see the current research efforts being conducted. Live patient trials are currently being conducted in France. The therapy basically takes stem-cells from the ALS patient, cleans them of the destructive material, and then reintroduces them into the bone marrow of the patient. The trials are just beginning in humans however; they have been widely tested in animals with promising results. The best hope for the future will be for Familial ALS descendants to harvest healthy cells and save cells from umbilical cords to be used in the future in the event that ALS surfaces in these patients. I lost my Mother, Aunt and Cousin to ALS; have one cousin with Parkinson's; and have lost 9 relatives to Alzheimer's Disease. I stay very close to the research on neurologically based disease.
At the moment, at least some forms of PD may be amenable to gene therapy. the cellular target is known and discrete, and the molecules of intervention may be assumed based on the gene mutations found in the patients (especially for loss of function). Early correction may block the progression of the disease and allows long-term treatment by pharmacology alone.
apart from the above answers , I would like to add a bit.
Currently a clinical method is being used called deep brain simulation. ( Just Google it to find more details). - It is one of the existing standards at the present scenario.
Research on gene therapy is still in the progress- may be another decade to see the results.