I've used Diamox on all four of my 14k+ hikes.
For acclimatization, they usually recommend starting at least 2 days before you start your high altitude hiking. (1 tablet twice a day). You only need to take it for 2-3 days in the days before your hike, then only need to take more if you start showing symptoms. You don't want to be taking it throughout a long trek if you're not having symptoms.
So far, only the 2nd hike using it did I have to abort my hike due to AMS symptoms. And I'm pretty sure that's since I didn't spend enough time acclimatizing. Just a single afternoon and night at 10,000 feet before going up to 12,000 feet on day 1 and then Mt Langley on day 2.
The other three, I was successful with my high altitude hikes. 2 x Whitney (including sleeping at the summit this past month) and 1 x Salkantay to Machu Pichu (with 16,000 and 14,000 foot passes and sleeping at 14,800 feet).
All three successful hikes were also with at least 2 days of acclimatization time, so the Diamox may or may not have helped.
At some point maybe I'll chance a high altitude hike without it, but I usually come down on the side not wanting to chance my hike failing and take it just in case it does help me.
Only side effect I've had directly from the Diamox is the first time I had some very nasty diarrhea when I started taking it. Haven't had issues with that since then.
I did have some diarrhea and long dizzy spells on my Salkantay trek, but those were after 2-3 days of not being on Diamox so I think they were general attitude issues or the Peruvian food. The dizziness was probably due to coming DOWN from high altitudes too fast (kind of reverse AMS).
