Didn't use it on my first two attempts above 5500m. Used it on my second attempt on Elbrus. While I did take plenty of time to acclimatize, I do have the feeling that I might not acclimatize as well as others. Started using a small dosage of Diamox after going above 4200m's on Elbrus and it did seem to help me.
In February I'm going to Kilimanjaro. I'm planning on taking a long time to reach the summit (15days). This should be plenty of time, but I will be taking Diamox with me just in case.