I second BB. You got to see what works for you.
I was using Grockit for my prep. Initially i felt using Grockit would help me to work under time pressure. I felt since each question is timed on Grockit it will help in coping up with the time pressure. But later i felt nothing can substitute the experience you get when you take the full length mock exams. That is a different ball game altogether.
Grockit was most useful when i wanted to practice questions on a particular topic. However, most of the questions i did on Grockit , i found them in OG 10
& 11. If you are using Manhattan series by any chance then you'll know that you get a list of relevant OG 11
questions on a particular topic at the end of each chapter. So in case you'd like to practice on a topic , you can use the OG too.I am sure there will be other ways to figure out the list of OG questions on a specefic Topic.
I used to like Grockit's videotaped explanations. But sometimes i felt they weren't clear. Also, for all the questions i found in the OG, the OG explanations were as good and sometimes even better.
I took their diagnostic test before i started my prep and felt it wasn't a good indicator of my skill levels. Grockit also estimates a score you could get based on the practice you do. I felt that score was very deceptive. Later during my prep i even felt that working under time pressure while practicing topics wasn't working well for me. I was only psyching my self out by using it while practicing topics that needed more time and was getting a sense of achievement by doing well in some topics.The point i'm trying to make is that such timing and scores are not a true indicator of how i performed during my full length mock exams, section tests or the actual exam. Later on i started using a timer only for section tests that i had got from a friend and used the GMATclub timer (it's pretty useful). Although section tests helped me a lot, i recommend that one take full length tests as the tiring journey from AWA to verbal can affect timing.
In summary, i 'll say see what works for you. If you'd like someone to read out the explanations to you and you prefer to work under time pressure for each question then Grockit could help. Some of the practice games were fun, where you get to play with others. On the other hand, there's enough study material available at better prices (sometimes even free
and in my opinion the full length tests are a great way to know how one performs under time pressure.
Hope this helps.