First of all I have to say that my initial goal was to score 700 points in the GMAT which I did not achieve. In my first, and so far only, official try I scored 650 after scoring not more than 580 in the test exams during preparation. After 2 months of classic studying with books I decided to go for the basic Economist GMAT Tutor option. I thought it might be more time efficient in order to support me for an intensive preparation in the last month before my official GMAT (and it was indeed).
I felt really comfortable working with the desktop version as well as the mobile app e.g. on my way to work. You can choose between 5 minute and 60 minute sessions on either quant or verbal or both combined. Tasks that were not answered correctly can be reviewed instantly and are noted by the software. Then, over time it gives you less of your strong tasks and more new topics with your weak spots frequently in between.
I started with a text exam right away and so the adaptive nature of the software got hold of my strengths and weaknesses pretty quickly. Obviously this is generally a huge advantage of software over classic book learning. Unfortunately I realized that far too late, otherwise I would have spend all 3 months of prep just with the Economist app. On the other hand I does make a lot of sense to work at least with GMAT material from two different companies in order to have a comparison.
Due to the insights from the test exam I focused solely on improving the quantative section, where the Economist app provides a massive amount of excercises. Although I spent three weeks only in the quant section I hardly managed to finish 50% of the quant section.
I am sure that the app it helped me to especially work on my weaknesses and that it deserves a huge part of the credits for my 650. Since I just got accepted to my first BS choice I can say that the 350$ were a good investment. They have different payment plans from which my option was the cheapest. It is valid for three months (one week free trial in advance) and comes with the study material + 3 test GMATs, 1 essay review, 20 ask a tutor questions via a chat interface.
Since then I have always recommended this app to anybody who asked me for GMAT prep recommendations.