Donald,
I had a good experience on my SAT Test Prep from ClarityEd. I found that learning from highly qualified teacher to be of real help, as the teacher can address all your weak points. Now I am working, but pretty soon I will be applying for my MBA and I will surely go back to ClarityEd for their GMAT Test Prep.
In case you want their website: http://clarityed.com/test-prep/gmat/
Jeff