I've had personal experience of using Diamox whilst in the Caucasus, on an trip up Mt. Elbrus.
Whilst several other members of the expedition suffered from varying degrees of altitude-induced headaches (at altitudes upto 18,500ft), I had no problems and found them useful.  As for side-effects, the classic is a tingling, pins and needles type feeling in your fingertips. This seemed to be with me for most of the time but wasn't so much of a problem that I had to stop taking them.  Another useful tip is to dissolve vitamin C tablets in your drinking water at altitude as it can help maintain your body's natural pH. which can be upset at altitude and lead to problems. 

Any how, on the walk in, I suffered with mild headaches, which after rest usually cleared, my mate had no ill effects at all. At about 6,000m high on Imja Tse, my mate had an horrendous headache and was showing all the early signs of HACE, I on the other hand was totally comfortable with the altitude and could have carried on.

The weather turned, and another member of the team began to feel unwell (he was also taking Diamox), so we decided to retreat.

Whether this says anything about Diamox, I don't know, but in my opinion you can't beat taking your time, and building into your schedule extra days, which will allow you to acclimatize properly.

If I were going again, I don't think that I would bother with Diamox, but I would extend the length of time that I spent on the walk in

I used it on a trip to Nepal when we only went to about 5,000 metres, mainly to counter bad headaches that I was getting - side effects were, as I recall, an increased desire to pee regularly (which is a sign of good acclimatisation normally anyway), and that was about it.

The second time I went to Nepal was to climb Mera Peak (6,470m ish depending on which maps you believe). I didn't take any Diamox that time, took it steadily and had very few problems at all except for one day when I overdid it and had a disgusting headache on the walk in (crossing a pass at about 5,000m), and on pre-summit day, when I had headache, inappetance and nausea camping at 5,700m. Throwing up over my wife's boots makes an entertaining story anyway!

Personally, I think Diamox may have some use at suppressing symptoms of altitude sickness over a short period, but shouldn't be used as a matter of course

take  125 mg twice a day before you climb up
there will not be severe side effects, but you will sleep better and will not have headache

Pack it but don't use it!
It makes most people pee like crazy...

But I hate taking pills anyway...

They say its only effective once you have been taking it for about 24 to 48hrs.  This means it is not effective as an immediate "relieve altitude sickness symptoms" fix.  Either take it for extended periods or not at all.

It is truly amazing how a small altitude drop can relieve symptoms - I only got bad headache once but dropping 50m did the trick

Did Kilimanjaro recently (just under 20,000 ft). Some folk used Diamox, others didn't, it didn't seem to affect the outcome as much as previous mountaineering experience and mental attitude (i.e determination to get to the top dispite headaches). I suspect some headaches were just dehydration/effects of sun not altitude sickness in which case Diamox won't help anyway. My GP who'd done Kili himself suggested take it but only use it if things get bad - I assume from this it must have some effect even if you only take it at the last minute. On the whole more Americans seemed to be taking it that others - I think this is probably just s differenet attitude to pill popping.

We always take Diamox along when going to altitude, but we keep it only for emergency use, rather than taking it as a prophylactic. Our attitude is that if you follow the rules of acclimatisation, then you shouldn't need any artificial help from Diamox. If you take Diamox, you can still get AMS (Acute Mountain Sickness). If you get AMS whilst using Diamox then you have already used up the possibility of using it as a cure! 
If you get mild symptoms of AMS you should stay put and not ascend. If the symptoms get worse you should descend without delay. In the scenario where you are unable to descend, (snowed-in, too dangerous at night, etc), then Diamox can be useful to tide you over until descent is possible. Make no mistake, AMS is potentially a killer, and much worse than pins and needles from Diamox.
 My advice, and this is based on many years of experience, would be as follows: Read up on AMS, so that you fully understand what is going on with your body. The subject is actually quite interesting, and you will be much more confident in recognising, and dealing with symptoms. Most Nepal trekking books have a chapter on the subject. Steven Bezruchka's trekking book has a particularly good one. Follow the rules, and keep your Diamox for emergencies. You are supposed to be having fun, not putting your life at risk. 
We have never had to use our Diamox on ourselves, but have had to give it to others on several occasions (my wife is a Doctor). We have seen many people suffer from AMS, and sadly seen a few people die from it. The key is to give yourself time to acclimatise, and keep yourself well hydrated, by drinking large amounts of fluids. You should be peeing regularly, and your urine should be light, and not dark in colour.

AMS is not something to mess with, and taking Diamox according to some folklore regime doesn't help.

we used diamox on our last trip to nepal cos work considerations meant there was little time to acclomatise. we galloped round the annapura circuit including thorung la in nine days with no trubble at all pins and needles just helped to confirm fingers were still there.

Personally I don't use the stuff - tried it once - and haven't have problems, but I suggest taking it along in case he/she is having problems, though even using Diamox, you shouldn't continue to ascend with AMS symptoms. Also vital to drink loads to compensate for the diuretic effects, though, of course, you should do this at altitude anyway.

The clinical trials indicate it should be used as a preventative measure, NOT as a cure. You start taking it 24-48 hours BEFORE hitting altitude. Read the manufacturer's instruction sheet, or ask your doctor to copy the pages out of the Physicians Desk Reference Manual.

There have been several posts about using Diamox, which is the trade name for acetozolimide (I am sure it is not spelled correctly) which is a carbonic anahydrase inhibitor. Diamox is normally used a renal diuretic and in normal conditions will make you piss like a race horse as a friend of mine says - this is a serious side effect as dehydration will make AMS worse.
 Other side effects are minor like tingling in the extremities and lips. (For some a major side effect is that is makes beer taste funny - a very good enough reason not to use it ;-)). 
I have used diamox, several times, but usually only for rapid ascents of more than 10,000 feet in a day. You are better off going slowly and letting your body acclimate on its own and taking diamox along just in case.

If you are planning on using Diamox - make sure you try it at base level first - the side effects vary vastly from person to person. Although the side-effects are listed as "tingling extremeties" - I found that I had a greater reaction with the whole of my arms and upper torso being affected.

First of all you don't need Diamox to go up Mt. Shasta (14, 162 ft.) by the time the shit is working on you you've probably gone home. Diamox is good for "High" Altitude, though there are side effects. I spent a week at close to 20,000 ft. and we ran out of Diamox before we even got that high, there were two of us and we suffered no ill affects from the altitude. In fact we both felt better without it because we were not tingling all the damn time, which can get annoying stuck in your tent in a godless storm for 7 days. Good Acclimatization is all that is necessary. Hydration, climbing high, sleeping low, rest days, etc.

I have used Diamox on several occasions. I usually find that within a few hours of taking it I'm pissing like crazy and getting tingling sensations in my extremities. I was under the impression that Diamox was a diuretic and prevented edema (pulmanory and cerebral) by draining fluids from your body. Hence it appeared to me that it was working very quickly.

According to "Medicine for Mountianeering", Diamox reduces the severity of symptoms (AMS) in individuals ascending from Sea level to 12,000-14,000 feet without the proper time for acclimitization. Additionally, it promotes acclimitization. No significant effect has been shown for HAPE. Preventitive dose is 250mg orally every 12 hours one to two days prior to ascent, continuing for three to five days after arrival at altitude.

They seem to work ok but they do dehydrate you, so you are up peeing all night, and they cause pins and needles in the extremities. I stopped taking them on the third day (ie I'd taken 5 - morning, night, morning, night, morning) because the pins and needles in my face were bugging me. Also after 3 days high I was acclimatised enough to deal without them.