i know you mentioned mgmat, but this applies to all gmat test prep cos (veritas, kaplan, princeton review, mgmat, etc)
situation 1 - you know the fundamentals of both verbal (sentence structure, grammar rules, idioms, etc) and math (add, sub, mult, div fractions, decimals, percents; how to solve an equation; dist/assc properties of numbers, exponents, radicals, etc)...
THEN YES, it is worth the gmat test prep online tutor - they can add to your fundamentals
situation 2 - you dont know the fundamentals of both verbal and math
THEN NO it is not worth it - look to brush up on the basics first by doing one of the following PRIOR to commiting money to a gmat test prep tutor (a) self study: take 15-20 hours to study math and 15-20 hours to study verbal - focus only on fundamentals by using a comprehensive study guide or book (b) take a class: esp a class that focuses on fundamentals (c) use another inexpensive tutor (like a hs math or english teacher or recent math or eng college grad) in your area to teach you the basics/fundamentals
i think these prep companies are good for prepping for the gmat after you build your fundamentals - that is how you get your money's worth. if you expect the test prep cos to teach you the fundamentals - that could take at least 10-15 hours (if not more) out of your package. and youll likely use 400-500 level gmat questions as your examples when initially applying the fundamentals. but if you want to target a 600-800 score, then get a good base before you go to a gmat prep test tutor.