Overall: I loved the Economist GMAT Prep. Before signing up I tried the free trial of Veritas and Manhattan Prep's programs and found that they focused on force-feeding you practice problems, whereas Economist GMAT forced you to slow down and focus on strategy and the fundamentals before jumping into anything. Overall, I feel that this strategy most strongly contributed to my success.
Economist GMAT had a relentless focus on strategy as a whole, and strategy within each question type. I never would have thought that I'd be getting so many difficult problems correct, or that I would be able to diagnose "lost causes" so quickly.
While learning all of the fundamental sentence correction rules and math formulas can be very boring, Economist GMAT's adaptive learning program kept me focused and moving forward. It helped me build a strong base that I'm not sure I otherwise would have been able to create.
Economist GMAT's explanations are second to none. Every question helped me understand the fundamentals, and their adaptive program recognized areas of weaknesses and forced me to spend more time on them. It also forced me to revisit questions I had gotten wrong in the past.
I do feel that I wish that Economist GMAT had an option to focus exclusively on advanced topics when I was "approaching the asymptote" rather than leaving the difficulty of practice problems to chance.