The next best thing after GMAT Prep

MGMAT CATs are probably the best non-official CATs out there.

1) Verbal questions are pretty good and represent the official questions to some extent. The one thing you would realize is that as you get better in your conceptual knowledge and patterns in official questions, you'd start doing good in MGMAT CATs too, simply because the MGMAT verbal questions are made using the official questions as a reference (true for any other non-official GMAT prep company).

2) Their Quant is definitely very calculation intensive, unlike GMAT Quant. BUT, it can still be helpful if you are targeting 49+. If you analyze the Quant questions and try to learn from every question, instead of focusing on how "hard" the question is, then you'd start realizing the value of their Quant questions. The main thing is to have a conceptual clarity of the concepts used.

3) My recommendation would be to take these tests in between your GMAT Prep tests as the GMAT Prep tests are limited and are closest to the actual test, and must be used wisely. Use the MGMAT Tests to pinpoint your weak areas, work on them and then take an official test to gauge your level/performance. These tests will definitely help you build the stamina for the real test.