It worked extremely well. I feel that I typically have a very hard time concentrating on learning new skills due to day-to-day constant context switching (I'm a Sysadmin/Devops by day).
I use Modafinil to focus and tCDS to boost my skills acquisition/retention rate. I haven't done any sort of study on myself to measure control vs tCDS time, but I definitely "feel" an increased rate of moving up the skills ladder (in this case, learning French, Spanish, and Russian).
For example, one of the potential drawbacks from focus drugs is that you lose creativity. What makes you sure your were getting a net win?
I'm very skeptical that there is really such a thing as a free lunch when it comes to synthetic cognitive enhancements.
That being said, I do believe physical fitness, diet, and adequate sleep seem to be massive cognitive enhancers when correctly applied.
I selected a provider (who ships from India) based on reviews from this sub-reddit. They accept my payments in bitcoins.
I haven't noticed a loss in creativity, but I think creativity in itself is something extremely difficult to measure or quantify. I attempted to join experiments being done by several labs across the country (with regards to tCDS), and was declined by all. It was then I went on to do it myself.
I agree with your last statement completely. I work no more than 8 hours a day, get at least 7-8 hours of sleep per night now, and eat in a much more healthy fashion.