I used to do a small amount of speed before my calculus exams. I would have one night of cramming. I never used speed for partying, only for studying and would only do it a few times a year. I would take it around 9 or 10pm and within 30 minutes start to feel the effects. I'd play online, talk a lot to whoever was around but eventually settle down into my night of studying.
I don't have ADD, but when i started to go through the examples and to other problems for practice, I could do them very well. For a short period of time the speed seemed to open up my mind so I could understand, I struggled a lot with math. But if I was on speed it wasn't a struggle. I would sit there and study constantly until it was time to take my exam. And almost always I would get an A. After the exam I would come home and crash though.
I have not done speed in a long time, its been almost 3 years. Im not suggesting that people study this way, it may not work for everyone. I didn't abuse it, maybe only 4 times out of the year.