I took the Economist Prep Course. I found it useful, especially for Quant.
Economist Mock 1: 770
Economist Mock 2: 770
I then appeared for the official mocks.
Official Mock 1: 690
This first official mock was shocking, as I did not do well in Verbal (especially SC).
For SC, I reviewed the OG in detail - line by line.
Official Mock 2: 770
In the real GMAT, I scored a 740 (Q50, V39). I was very poor with my time management. I could have done better.
That said, I am not reappearing. Overall, I would say the Economist GMAT Prep Course is pretty good.
P.S.: My quant was decent even before I started preparing. But I found their tips to be pretty useful. In verbal, Economist's RCs are tougher - crazy tough. SC is just ok (one has to go through OG in detail). CR is also decent.