I have started using this 2 years ago and bought a paid subscription for mocks a year ago.
The first thing I have to say is the quality of mocks is really good. Unlike other apps which either makes is too difficult or too easy, Testbook provide a balanced mock with good analysis. Previous year papers are also available on this.
Second thing they offer is online coaching via membership of testbook select which is of no use. Teachers quality is not up to the mark. Testbook is focusing more on quantity rather than quality which is very bad thing for students.
If you ask me I will recommend it only for mock at good price, testbook select is not good.