My own experience :
21 yo, French, BSc in Management, will apply for MSc in Marketing in Europe.
Without any prep :
Kaplan Diagnostic : 600 Q38 V33
With prep (chronological order, all one week before the test) :
Kaplan CAT 3 : 540 Q31 V29 (I was so desperate before I found out this wonderful site)
Kaplan CAT 2 : 540 Q31 V28
Crack the Gmat : 620 Q44 V28 (Feeling a bit better)
Kaplan CAT 1 : 570 Q33 V33
GMAT Prep 2 : 640 Q44 V33
GMAT Prep 1 : 710 Q49 V40
Real Score :
690 Q49 V34