I just came back from the test center and I am happy that it's done. Since GMATClub was a big help in my prep, I wanted to share my experience and the material I worked with.
I started with the
Magoosh videos. They had them on sale when I started including a 7-day money-back guarantee, so I took it. The videos they provide on their platform where incredibly helpful to build a foundation. I have a masters in physics already, so technically, the math was a no-brainer. However, it was still great to walk through all those little shortcuts and tricks that separate the GMAT from an actual math exam. In some parts, especially in the quant section I found that the videos are a bit too practical, focussing on solving the question, without understanding the underlying theory, e.g. by simply picking numbers. However, that was just my feeling, I can imagine that that approach works better for people without a strong math background. The verbal part of their videos was fantastic. English is my major language at work, but I never really learned it in the formalised, structured application, that you need for the the GMAT, so those videos were incredibly helpful. Even during my later work, I often went back to those and used them whenever other study material just brushed over certain questions without really explaining the grammatical structure.
I then took my first prep test from Kaplan and scored 760 (Q51,V44) in mid December. That would probably have been enough for the GMAT, but I enjoyed working on it, so I kept going. My wife was out of the country in December and January, so I had our place just for me to focus on my prep.
I worked through the Kaplan Premier 2015 and then the Kaplan 800 books. That kept me busy until just after christmas and in between I took a few prep tests:
Kaplan 1 750 (Q51,V44)
Veritas Free Test 780 (Q50,V47)
MGMAT Free Test 740 (Q50,V41)
Kaplan 2 760 (Q51,V45)
Economist 720 (Q50,V40)
I felt pretty confident before I took the Economist test and probably would have taken the real test by mid Jan, but the last one made me doubt again, so kept on going. I got the Veritas books from a friend and used the free videos lessons from their iPad app to study further. Those videos were again very helpful, because they focussed more on fast solving methods and strategic approaches than the
Magoosh videos. While going through those books and videos during weekday evenings, I kept on doing prep tests on the weekends:
Kaplan 3 760 (Q51,V45)
Kaplan 4 740 (Q50,V42)
Kaplan 5 750 (Q51,V44)
Since those where again around the same level that I had scored before the Economist test, I felt more comfortable again and scheduled the exam to 6th of Feb. I spent January working with the
MGMAT Advanced Quant book to focus a bit on Quant strategies, although this was now only cosmetics.
In the last two weeks before the exam I went through the last 30 or 40 questions of the PS, DS and SC sections of the OG12 and did the two official tests, which I scored 770 and 780. However, that last 780 did not really count, because I knew around half the questions in the second exam from the first. Did anybody else have this experience, or was that some bug in my installation?
Throughout the entire 3 months, I was actively reading all the GMAT blogs I could find, such as Veritas, Kaplan, Economist, Manhattan,
Magoosh, etc. All that helped me to completely dive into the GMAT and understand the idea and concept behind it. In between free online lessons that Brian Galvin and David Newland from Veritas offered occasionally throughout those three months were very helpful, though I had to get up in the middle of the night for them. Brian and David, if you read this, please consider offering lessons that fit European times better

Furthermore, since two years, I am subscriber to the digital version of the Economist, which comes with an audio version of all articles and also helped tremendously in the entire verbal section. I have lived in China for a long time and found that if you select a Chinese home address for the subscription, the annual fee is just USD 52, so consider signing up as well.
For the first 6 weeks of my prep I used an
Error log as that was recommended here. I guess it might be very helpful for some people out there, but for me it didn't turn out to be useful. Through the entire period that I worked on test exams and questions, I never had the feeling that I had systemic difficulties with one question type or the other, but most of my mistake were related to reading through the questions too quickly or not trying to pre-think before actually looking at the answers. Especially the latter technique was very helpful for me and I can only recommend it.
The last weeks were really tough, because I hardly went out during that time and spent most of my evenings studying, so I am happy that it's finally over.