I studied for the GMAT in 2017 with a private tutoring package for 20 hours. Gregg Lachow was my tutor, and we met exclusively online via Blackboard. I wound up scoring a 760, up from a 630 MPrep CAT before the sessions began and an official test score of 710 back in 2013. I had four scores in the low 700s in official tests during my MPrep course before I scored the 760.
Private tutoring worked for me because I had a few specific problem areas with the format (timing, test anxiety, rushing) and content (quant and main idea questions in verbal). Every tutor is different, so I can only speak to my experience with Gregg and the MPrep materials.
I used MPrep study guides including "Foundations", the scratch pad, Interact, and all six of the online CAT. I also used the OG, "Verbal/Quant Review," GMAT Prep, GMAT Focus (a sort of online simulation of the quant section), and the GMAT Club forums. So I pretty much availed myself of every scrap of prep material out there in the world. In total, I probably studied for a few hundred hours over the course of eight months on top of applications/work. Manhattan Prep practice and CAT problems for quant were more difficult/discouraging than those I faced on test day and its verbal problems slightly easier. I think that the most helpful guides for me were the "Foundations of Math" and the "Sentence Correction" guide. Overall, the CAT scores did roughly track to my actual performance although I think that the GMAT Prep ones were more lifelike. Interact is fairly useful for getting the big ideas and practicing the common problems, but it may not be the level of detail needed to score in the 99th percentile.
We began the tutoring sessions by reviewing my study habits, target score/timeline, prior performance, and test-taking habits. I liked the overall organization of the entire study background/plan/schedule/notes in a single shared Google doc.
With Gregg, I covered each section/topic of the GMAT over the course of about a dozen 30min-1.5hr sessions. Each session was recorded and saved, so I could concentrate on the material and not obsessive note-taking. After each session, I would watch the video and jot down the key takeaways. I was skeptical of online tutoring at first, but I'm glad I signed up for them just for the recordings. They were invaluable.
Gregg is a wiz: he knows the test inside and out and can walk you through tough questions with aplomb. He's good at making sure you nail the execution (process, process, process) rather than rush to the right answer, even for areas of relative strength. He breaks down the test into the smallest pieces (from fully understanding the question to jotting down the givens and understanding how the test makes problems hard). He not only teaches the concepts, but also explains how the test can complicate those simple concepts with traps. If you find yourself falling for "the right answer," then Gregg can help you correct these ingrained habits. He also can translate "GMAT-speak"–the sometimes overly-specific and non-everyday meanings of keywords–into ordinary English. And he excels at forcing you to take a step back and approach the test and each question in the right mental/emotional/intellectual way.
Although he's stronger in verbal, especially sentence correction, for quant he excelled at showing me how to spot common traps and tricks that I couldn't via independent study, explaining the intricacies of CAT scoring and how they relate to skipping/timing, and providing tactics for every single problem type (not just DS/PS, but rate/work vs. geometry vs. statistics, etc.). If you have nailed quant, then I'd say he would make an excellent standalone verbal tutor as well, with perfect SC accuracy and an eagle eye for RC/CR pitfalls.
All in all, he both prepared and coached me to perform at my peak level on test day. I definitely recommend him as a tutor because I wouldn't have hit my score without him.
The folks who work in the offices are friendly, and the customer service is excellent. The website is somewhat confusing to navigate because there is so much content, but overall has amazing resources and no downtime. They even threw in GMAT Navigator (OG answer guide) for free, which was nice.