mugdha
Hi,
Some of my friends who have already given GMAT are suggesting me not to study any other author besides the official guide and official guide verbal review or official guide quantitative review. Do you think i should really consider this advise seriously?????

I would not doubt the intentions of your friends. Certainly, no one can do without OG. Whenever you take a test, you do go through the past papers, don't you? For some people, it could be sufficient. Say, if you are a Quant jock and are pretty comfortable with concepts of arithmetic, algebra, geometry etc, all you may need is checking out the type of questions specific to GMAT. If you are very comfortable with English and have been doing RCs since school time, you may not need anything other than the Verbal guide and OG12. If you know your grammar well and instinctively know what is right, what is wrong, you may be good to go.
However, OG is not written to help you prepare for the test. It is written to show what kind of questions are asked in GMAT (or were asked a few yrs back in GMAT). If you feel you need to go through the theory of mixtures or TSD or Inequalities or CR etc, OG will not be very helpful. You do not get detailed theory of various topics in OG. You do not get many high level questions in OG (the actual test is much harder now).
So if you feel that all you need is some practice to get rolling, by all means stick with OG. After all, it is closest to what you will see on the test. But if you need concept clarifications, critical reasoning explanations, many high level practice questions etc, you will need to check out some prep provider's material too.