You've asked a very serious question about DBS surgery post operative results, side effects and effective duration of this therapy. The question is of course, what is the time frame? 5 years, 10, 15, 20? There isn't much clinical evidence published with these answers.
The fact is that among the patients who have successfully gone through the process. There are many glowing anecdotal reports of wonderful results even when there have been complications. Those people are probably in the majority. After the surgery a period of a few weeks pass when the "system is being calibrated." If there are problems later, recalibration can be done. Which means that if tremors were controlled after adjustments, they should continue to be controlled. There are just such positive reports about readjustments in the event of tremor return at Patients Like Me.
It is not the tremor return that post-DBS patients have primarily complained. There are patients who for one complication reason or another found themselves in either the same place or a worse place than before the surgery. One of the biggest issues is in cognition which shouldn't be expected to improve and in fact may suffer. A number of patients develop depression, dementia, balance issues. However even those with balance and depression problems would go through the procedure again. And many might if they only had unilateral surgery done.
It is important that people understand that there are risks to this surgery just as there are risks to any invasive procedure and that not every patient is a good candidate. The person must be levodopa/carbidopa responsive.
One thing that is important to know is that even with the newer batteries which prolong the life of the device, when those have to be replaced, some health plans do not cover the replacement surgery (which makes no sense when you think about it) and some do not cover the cost of the battery which is not like replacing a battery for the smoke detector.
The DBS primary function in PD is to aid Tremor and Dystonia and seems to have an a sustained positive effect on those symptoms once the proper adjustments are made.
Here are some links for additional reading and consideration:
Ethical problems - increased risk of developing dementia
http://www.jaapa.com/ethical-issues-surrounding-deep-brain-stimulation-in-parkinsons-disease/article/127598/
"Pre-existing dementia is currently considered an exclusion criterion for DBS surgery. However, patients with PD are at an increased risk of developing dementia over time. Presurgical evaluation is used to screen patients carefully, so that DBS is not offered when the patient's expectations are not consistent with what DBS can offer and so that patients are educated about possible benefits and risks. The surgical risks of DBS include cerebral hemorrhage hardware infection, suboptimal placement of electrodes, wire fracture, hardware erosion, apathy, suicide, eyelid opening apraxia, and weight gain. The risk-benefit analysis is unclear when considering the above risks in a patient who has dementia or is at high risk of developing it."
An interesting article which claims that DBS halts both hair loss and the loss of dopamine cells
http://www.sciencedaily.com/releases/2008/09/080902171151.htm
From Medtronic the primary appliance company:
http://www.medtronic.com/your-health/essential-tremor/important-safety-information/index.htm
"the abrupt cessation of stimulation for any reason should be avoided as it may cause a return of disease symptoms. In some cases, symptoms may return with an intensity greater than was experienced prior to system implant (“rebound” effect). Adverse events related to the therapy, device, or procedure can include: stimulation not effective, cognitive disorders, pain, dyskinesia, dystonia, speech disorders including dysarthria, infection, paresthesia, intracranial hemorrhage, electromagnetic interference, cardiovascular events, visual disturbances, sensory disturbances, device migration, paresis/asthenia, abnormal gait, incoordination, headaches, lead repositioning, thinking abnormal, device explant, hemiplegia, lead fracture, seizures, respiratory events, and shocking or jolting stimulation."
DBS - Risks and benefits: "Side effects, if they occur, are usually reversible, but may include weakness, speech and swallowing difficulties, and abnormal sensations.
There are risks associated with any brain surgery, including infection, stroke, hemorrhage, cerebrospinal fluid leaks, seizures, weakness, tingling, technical problems, wound healing problems, disfiguring scars, prolonged hospitality, and need for additional surgery. The percentage of patients who report these complications is low.
In addition, there are risks associated specifically with DBS, including speech difficulties, such as speaking softly and slurring of words, mood changes (especially depression), involuntary movement, difficulty with coordination, recurrence of symptoms, malfunction or breakage of device, device migration, and early removal of device. Adjustments of the stimulator and speech therapy sessions can help to reduce some of these difficulties."
http://www.bcm.edu/neurology/patient_education/pdcmdc/dbs.html
Adjustments can be made later - finer targeting will be developed and are in research now. Just remember that DBS is not a cure and the disease will continue to progress. But the improvement in quality of life is immeasurable for so many people who have undergone the procedure - some more than once as the disease lays claim to the other side of the body.
Addendum:
I just found this old abstract about a followup study of 9 unilateral DBS PD patients. Tremor scores were still significantly improved from the baseline about 40 months after surgery.
http://jnnp.bmj.com/content/71/5/682.abstract
And one other interesting article about electrode allergy and other hardware issues - there is a reference to tremor control:
http://www.neurologyreviews.com/may05/DBS.html
http://jnnp.bmj.com/content/71/5/682.abstract
So the answer is that while tremors can return, they can also be recalibrated out of the picture.
About having the surgery, even the people at PLM who had problems were happy to gain back some control of their lives. One person at Patients Like Me observed that there is risk in everything but if you want to get to the other side of the street, you cross. .