The service we used was NOT a national one; it is local to the Baltimore-Washington DC corridor, and it has been in business since 1997. I think that one of the most valuable help that these test preps provide is ‘forcing’ the students to study in a structured format, and the second most valuable aspect with the service we used was that all material were printed, thus reinforcing the real testing condition. I remember joking with my husband that we would save money if we could tie the son to a chair and make him do the tests for ten Sundays.
I think that any student who study regularly and earnestly, with or without test prep, can do well (not bomb) on these tests, young brains do adapt very quickly. Another piece of unsolicited thought: we also used the essay/application writing service from the same company. If we had to choose between using either the test prep or the essay service, I would choose the essay service.