I am using e-gmat. Please note that i am only half-way through with this course.
Positive :-
1- The emphasis is on understanding the intended meaning before you proceed to resolve SC questions.Also, how to tackle SC problems such as breaking down SC problems in to clauses.
2-i have found till now is that questions related any concept or questions from OG get answered in 24 hours with detailed explaination.
3-There is a pre and post assessment quiz after each tutorial.
4- You can first create a login and use few sessions before you decide to buy. ( if it does not fit your style then don't buy).
5- If you have more doubts then i suggest contact e-gmat. I also remember that there are 2 one to one sessions.
6- There are almost 100 gmat style problems discussed in detail (please check site for correct number).
Negative:-
1. You can possibly learn all these things from Manhattan SC
2. There are no 6 gmat tests (that you will get with Manhattan SC)