To give an opinion that hasn't been offered... I am diagnosed ADHD.
I have a hard time focusing and concentrating more than 10 minutes at a time. I am a rising junior at my university, and have a 3.94 GPA. I am not prescribed adderall, although I may be soon, but for a long time have not had it. It has been an extremely difficult struggle to work for my chemistry department, study so hard with so much effort, and try to live a healthy, mentally sane life.
I use an extreme amount of caffeine, about 500mg to 800mg a day, sometimes less, sometimes more. I cycle it to help its effectiveness/stay somewhat healthy(caffeine isn't really great for you in the doses I have to take it at.)
I know many friends who have taken and abused adderall. The point is, you can take adderall, but the worthy will get what they deserve. Work hard at something and do your best. If you feel like you may be ADHD, genuinely start to take a look at yourself and your behavior. ADHD isn't something you develop. Its a mental condition(I am hesitant to say disorder) that puts you apart from a lot of smart students, mainly because people with ADHD often manifest their time in other things than pursuing scholastic endeavors, or aspiring to go to medical/dental/graduate school.
To give you an idea of what I have to do to study...
I have to have complete silence. Music, talking, or any irregular sounds that aren't specifically in tune with my train of thought distract me where I lose focus. I do best studying when I have someone with me, studying the same material, that I can practice/go-over with them. It always helps me keep my mind from wandering because I see their physical presence as a duty to make sure I am studying along with them. I am extremely hard on myself and I want to succeed. This is what makes me get the motivation I need to keep studying. Every time I lose focus, I have to muster up motivation in order to try my best to stay on task. This is very stressful for me, and has proven to be very hard emotionally since I have been in college, mostly because I am extremely hard on myself(and have to be, to maintain focus for 5 minutes, and to stay on track to do my absolute best in all my classes.)
Even when I do study, it is extremely difficult to keep my mind in one place. As I said I use caffeine and it helps me bring together my train of thought some, but it is not magic, as millions know. It wakes me up and thats about it. I have used caffeine since high school( I was drinking six cups a day my junior year, when I was taking chemistry 2, so I could do well in there.) To anyone reading this, it is very obvious I was self medicating my ADHD. Adderall is the same way. I see people take adderall and do nothing productive and just talk/play video games/get on their phone. I also know people who use it and benefit well from it, whether they only use it when they study, or whether they use it every day as prescribed in order to live a more functional life.
To the OP,
If you are considering taking adderall illegally to study, by all means, do what you think is best, BUT there is a good chance you DO NOT need it. Like I said, I have come as far as I have not having it, and I am diagnosed ADHD. To give you an idea, I was brought up by an extremely scatter brained parent, who has a hard time putting anything together mentally. My mother, as well as myself, often avoid mentally challenging tasks because they are honestly difficult for us. Folding clothes, cooking, tidy-ing up, have always been extremely difficult. On the flip side, she and I can both get engulfed in something for hours that has no relevance to our lives at all("hyper focus"). I often research things online to such an extent that it would shock 90% of the people who are still reading this post. - To end this post, I just want to say, I thought ADHD was an excuse people used to get their kids to stop acting hyper. I always thought that it was a joke, even in high school, and I felt sorry for all the kids taking ritalin because their parents pushed it on them if they weren't 'little einsteins'. Now, after a deep look at my life and behavior since I was a child, to my schoolwork, etc., I came to terms with my ADHD(through a psychiatrist).
EDIT: I just realized this thread is from 2008.. I will keep my story in here though, in case any one else should happen to read it.