The first time I ever packed Diamox in my mecical kit was when I went to Bolivia, in 2009. Before that, I had climbed just one peak over 5000 m (and had a mild headache while doing so), but now my sights were set on several peaks over 6000 m.
The plan was not to take Diamox preemptively, but carry it just in case I would find myself in real trouble with AMS on a mountain, in which case I would take it just to be able to descend safely. Right before traveling to Bolivia I had been in the Alps and as a result of that, fortunately, I had no altitude problems at all. Sure, I was a bit slow at first, but after three weeks, I could even run in La Paz (ok, I'll admit it, it was just a short, slow jog).
Then, last December I went to Ecuador and I carried it again. This time I came straight from Amsterdam, had less time and rather ambitious plans and as a result I woke up with a mild headache for the first couple of days. Paracetamol did a good job in suppressing that. However, by the end of the third day my aggressive schedule caught up with me. The headache returned during the day and didn't even go away when we descended, I had no appetite and felt generally poor. Of course, that doesn't necessarily have to be altitude sickness, but when in doubt, I understand it's safer to assume that it is. The next few peaks in my plan would be higher again, so I decided to start taking Diamox preemptively and see if it would help. The prescription read"half a pill in the morning, one pill in the evening, and continue for three days after the symptoms are gone".
The next three days I still woke up with a mild head ache, for which I took paracetamol in addition to the Diamox. After that, I didn't have any problems anymore - even though it still took a couple more days before I wasn't slow anymore.http://www.summitpost.org/obstacles-on-the-ilinizas/690707
Despite this success, in the future, I plan to reserve a bit more time for acclimatization, so I won't need Diamox, but just in case I don't have enough time, I now know that it works for me.