I would concur with everything Keif says here - I've taken dexedrine for many years, at one point up to 40mg/day IR. I even started supplementing this dose with stuff like cyclazadone, ppap, and then even the designer amphetamines like 2-fma. I ***needed moaarrr stimulation*** and even though I would never have admitted it at the time, I was chasing. Some nights I would come home from work and just worry about my heart, have some drinks (or do some G) until I could pass out or take an ambien - rinse and repeat.
I eventually had a moment of clarity, I was very clearly abusing this stuff, not getting much benefit, and it was ruling my life in a way that I felt helpless around. I decided to just stop taking it, switched to a mixture of l-tyrosine, bupropion (welbutrin), and N-Acetyl-Cystine for about a week or so, to off-set the withdrawal syndrome (which was actually fairly effective). I then spent about 8 months off of it. I found after a while that something was lacking as I legitimately struggled to keep up with work demands and my mood was frequently low etc. I decided to revisit dexedrine but this time I decided that I wouldn't go above 20mg and I wouldn't use IR formulations.
I find XR to be much less prone to compulsivity, and I find that 10-20mg is honestly ideal for me. If I start taking higher doses than that, I tend to just get more edgy and have a harder time with task-switching. I also like to take lower doses on the weekends, vacation, holidays as it lets my mind 'downshift' for a time, and lets me build up a little cache of supply in the event I want to binge for a night or if pharmacy supplies are disrupted (which happens from time to time).