See, ,,most of the experts will advice u to take a gmat diagnostic kind of test , and most of these advisors are ppl who run gmat classes....,, so u take a diagnostic , get scared of ur ''outstanding'' scores and run to these classes...!! lol...

so basically i am very much against the point that ppl should give a daig test even bfr they start their prep...

Reasons:-

1) In maths, most of the concepts are very basic. But we have forgotten them,...since school was ages ago,,so u can atleast brush up these topics and effeciently score 50 points higher...
(thats my take)

2) In verbal, apart from reading comp, gmat prep has very specific questions in SC and RC, so a review and understanding of these nuances, will significantly alleivate the score...

Why I am saying this is because when I started studying, I realised i had forgotten so much, and if i had given a diag there and then , it would suck..

But now , I am pretty confident of scoring a 700+

SO .....My advice is, dont rely on the diag, and pls dont waste the GMAT Prep test , they are invaluable...

Set up a plan , study for 2 months,,then take a test, which i would call here a "diagnostic"--one that would tell what you are capable of.... find out ur weaknesses and ur strengths, work more on ur strenghts and try to avoid making the mistakes u made bfr ...all this in probably the last few weeks before ur test...

And ya, last point, take a test date,thats sets a target,,!!
then hit it Bulls EYE..!!

Best of luck with ur prepping