Just back from Nepal.
Well if you think that you are prone to AMS that recommended start for Diamox theraphy is at 3500 meters (prophylactic approach). They come in 250mg form and you should be taking pills one in the morning one in the evening. I never took Diamox (multiple times over 5000 m) but a lot of my friends did and I carry it with me anyways since there is no rule on who will get the AMS. More common approach is to start the theraphy when you start experiencing mild symptoms of AMS (light headache, etc). If you start experiencing serious symptoms of AMS it is too late to take diamox and you should descent. Diamox also should not be used as a "safty pill" meaning pushing to hard and taking it to fight of symptoms.
Diamox is a diueretic (like coffee) and will keep you dehydrated. Also that means frequent trips to the bathroom; not a problem during day but problem while sleeping. Ever tried getting out of the tent at 6000 meters, 7 times to pee at 0 degrees? Buddy of mine record is 15 times in one night. Makes it for bad night and crappy climb the next day.
If you follow general rules and use acclimatization days and carefuly calculate your ascent rates I don't think you will need Diamox. A lot of times I see people who push themselves 1500 meters up in one day make too big jumps and wonder why they cough up bloody foam. My girl who was with me on this trip and it was her first high altitude adventure followed all the rules and had no problems. Just remember go slow, like a yak
This may happen if you don't take diamox (Polak on the left, Cho La Pass at 5000 something in Annapurna Region Nepal):