Hi there! Just wanted to share my experience on how taking practice tests online helped me get a decent score on the GMAT.
With about 3 weeks left for test day, I started attempting practice GMAT sample tests. During the first couple of weeks, I skipped the essays and directly moved on to the main portion of the exam. Even though some common questions popped up in successive exams, I made it a point to note the tricky questions especially in sentence correction and data sufficiency. In this fashion by retaking the practice test many times, not only was I able to time myself and measure my performance against previous attempts (after adjusting for repeated questions) but I also got the much needed practice to tackle questions which are framed (i.e. with similar wording and language style) similar to what appears on test day, This is one very strong advantage that one gets by taking the GMAT practice tests in preference to other sample tests.
Moreover, taking the GMAT exam can be a very tiring experience because a) The test is pretty long b) Your eyes will be strained at some point or the other. As I wasn't feeling well by the last half an hour of the exam, I couldn't carefully analyze all the options. I'm certain that my prior experience with the GMAT style practice tests helped me predict the answers in the English Section by simply associating the question with a similar type as on the practice test. It is especially in this sort of a situation that I feel that my experience with practice tests came in handy.
And yes, the proof of the pudding is in the eating