720 from 590. David Mahler - The best.

Got my target 720 score after approx 4 months that comprised self study, Manhattan Prep's 9-week course on Wall street Manhattan (David Mahler was our instructor), and private tutoring (also with Dave Mahler). I started with a 590 Manhattan Prep GMAT practice test. Quite frankly, I could not have managed the 720 without Manhattan Prep.

1. The 9-week course: This was a great start for me as my math skills were very rusty (haven't touched math since high school and my day-to-day work doesn't require the use of any meaningful quant skill - I'm a lawyer). The math foundations and GMAT test strategies explored in class were concise and immediately helpful. Dave Mahler was our instructor - I highly recommend him for the clarity with which he explains tough concepts (particularly Data Sufficiency and Critical Reasoning). FYI the 9-week course comes with the full suite of Manhattan Prep Books, which were all great - concise, clear, and effective - no verbiage which is good for busy professionals.

2. Private tutoring: After the 9 week course, I spent a couple of weeks wrapping up the GMAT OG and then I took a real GMAT test. Bombed it. I decided I needed more targetted tutoring, so I splurged on the private tutoring (8 lessons in total). This was worth every penny. Dave diagnosed my flawed test-taking strategies, and his private lessons sharpened my quant and verbal reasoning skills. These lessons were challenging but crucial to my success on the test - he would often ask me to explain my thought processes WHILE I solved tough problems (DS, PS, SC, CR), and this invariably forces you to think critically about every single step in your own problem-solving process. Every step in my problem-solving process was critiqued and improved upon. As important, Dave was extremely optimistic and encouraging throughout this whole process. He was a firm supporter and you feel that he's invested in your success.

TL DR: Manhattan Prep's 9 week course and private tutoring are both very strongly recommended. Manhattan Prep's books are very strongly recommended. Dave Mahler is very strongly recommended.