100mg of 60% amphetamine is 60mg of pure amphetamine, which MAY be quite unpleasant for unexperienced, tolerance-naïve, not gargantuan-sized person.
A friend of mine takes 40-50mg of Dexedrine (pure dextroamphetamine sulphate) per day - it was initially less, but after over 5 years of daily use, this is his dose. You will notice that a person that has taken this for years daily still takes less (in 3-4 separate doses, vs. all at once) than 60mg. That's not to say he couldn't easily handle 4x that dose without getting fucked up, but people seem to think speed is less potent than it is - probably because it's so notoriously adulterated.
The first time another friend took amphetamine (Dexedrine also, so we know the purity) 15mg kept her euphorically dancing all night (though it must be said she was about 5'3" and inexperienced with stimulants).
Now, street speed will also contain the levorotatory isomer, which is far less potent than the dextrorotatory, so 100mg of 60% pure street speed (ie. 60mg of racemic amphetamine) is not the same as 60mg dextroamphetamine.
If you're sniffing it, start with 50mg. 100mg won't be dangerous, but some people already risk overstimulation at such a dose. If you're not satisfied in 15 minutes, do another 50mg. It comes on fast, but there's no "rush" or massively developed tolerance and diminishing returns - a second dose administered 15 minutes after will not be a feeble attempt to chase your high from before, but rather a bump up.
Taking too much, too much, is certainly unpleasant, but usually quite safe - you may just have 30-180 minutes of feeling like absolute shit, trembling, sweating and puking.
I'm noticing that "100mg as the standard unit dose for amphetamine" is a myth that seems to have quite a following, and I hate coming across it. If the amphetamine is pure, 100mg is a lot. If it's crappy and you have tolerance, 100mg could be fuck all.