Not trying to crusade against people using insulin for bodybuilding, but thought i might share a personal experience of mine so people can make a balanced judgement.
Firstly, i am diabetic (type 1) and have been for the last 8 years, so i have to use insulin injections, not by choice. Lately i have been intentionally upping my dosage after gym to enhance muscle growth, but i do so remembering something which happened 5 years ago . . .
I woke up at 6am and tried to get out of bed. As i stepped out my, left leg gave way entirely and i fell to the floor. I tried to lift myself up, only to find that both my left arm and leg was entirely paralysed. I tried to call out for help (I was living at home with my parents at the time) only to find that i couldnt yell (or talk for that matter!) I was able to with my right arm grab a bottle of powerade which was on top of my football bag and drink it to reverse the hypoglycemia.
Needless to say, it was probably the most frightening experience of my life and I would never wish for me (or anyone else) to relive it. Not being able to physically talk was terrifying.
Moral of my little story . . . if your heart is set on using insulin for bodybuilding, make sure as hell you can monitor your blood sugar levels and avoid longer lasting insulins (protophane etc) at night.