As an interviewer of applicants to London Business School, I'm often asked how important the GMAT is. Of course, it's very important.

A great example of this is the GMAT Validity Study. There's one here: http://www.gmac.com/~/media/Files/gmac/Research/validity-and-testing/RR0506_VSSSummaryReport.pdf The study shows that the GMAT Total score is the strongest single predictor of success in MBA courses, and added together with the undergraduate GPA is an even stronger indicator.

You can read more at http://www.gmac.com/market-intelligence-and-research/market-research/research-report-series/gmat-validity-studies.aspx