I have had two of the devices that are used to treat Parkinson's implanted in my brain for the past 8 years. Its currently been removed in favour of the deep brain stimulator which is also used for Parkinson's treatment. I suffer from cluster Headaches and am in pain 12 hours of the day and when the implant is switched on the pain goes away. Its the same for Parkinson's - switch the device on and the tremors disappear. I met someone with Tourette's who switched it on for the first time and to see them stop ticking was amazing.
One problem is something called Neuroplasticity - where the brain is clever enough to route around the implanted electrodes and thus return the original symptoms.
Some interesting things used to happen - I could bring down Wi-Fi networks by walking into a room, I used to set off those shop security detectors and all sorts of weird stuff I couldn't prove was the device but it was too much of a coincidence to dismiss. The device manufacturer was always disinterested ijn any feedback.
The later models had their own IP Address which was a way to get the data off remotely rather than by the Neuro programmer which had to sit on the patient's skin. This was a concern and was how the heart pacemaker was hacked in the TV Show Homeland. The brain version wasn't producing anything that could kill someone so was only a minor security issue.
I am waiting for the deep brain version - approved for past 3 years but NHS have run out of money.