I've taken all three before. Although I've on Dexedrine XR for a few years straight now, so most of my experience is with enantiopure (d)-amphetamine. Compared with Adderall, pure dextroamphetamine is actually less stimulating--as most people will attest, it is smoother and (at least in my case), has an almost anxiolytic effect. Higher-echelon doses of Adderall really engender a laser-focused, 'wired' feeling, as mentioned. Modafinil is not as useful for ADD, but is excellent for getting tough times that require one to get less than a desirable amount of sleep. Methylphenidate makes me kind of tired and lost its beneficial effects after a while. Not to mention the short half-life is extremely annoying--it is easy to forget to take it on time and then the 'rollercoaster' effect sets in.