The biggest issue is that you're artificially screwing w/ your response to carbs and if you don't get a sufficient carb up in there at the right time, it can introduce some siginficantly scary problems. When I say "really, really, really, really know what you're doing" - it goes to being extremely experienced and disciplined in your scheduling, your dosing, understanding and managing your body's response to it. The purpose of using insulin is to sort of artificially replicate how insulin works in your body by introducing exogenous insulin. But you also need to manage your body's response to it. Basically a lot of the things that people use in bodybuilding to improve results are to try to replicate a particular body mechanism, isolated and enhanced, to get the desired result -but just as w/ using AAS to grow muscle, you never get to pick and choose the results & sides you want - you get all of it. W/ insulin you also need to accommodate your body's response to the presence of insulin, just as if it were producing it itself. I doubt anyone, including the best doctors, have a complete understanding of the full effect and dependence of any one function in the body - so its a crap shoot when you're self-medicating. There's always the chance of too much / not enough / too soon / not soon enough of any one thing that is part of the overall push / pull biochemical balance the body relies on to maintain homeostasis.
I was looking for a short description of it and the dangers of it but didn't really find it quickly - so I'm pulling out two statements from livestrong.com that summarize:
Why use it?
Insulin helps to volumize each cell. This is why insulin is considered to have an anabolic effect. Bodybuilders take it to increase stamina and improve endurance. Additionally, it is used to increase muscle bulk because it stimulates glycogen formation, which feeds the muscles during a workout. Finally, insulin works to prevent the breakdown of muscle protein, which also helps a person bulk up with muscle.
Considerations / Dangers:
High-glycemic index foods such as jelly beans, honey, glucose tablets, maltose, or dates are consumed with the injection to prevent the blood glucose level from going too low. In addition, a very rigorous diet must be followed to see the benefits and to decrease the risks involved.
Improper use of insulin can lead to the person developing diabetes mellitus, long-term organ damage, coma, or death. Symptoms of insulin shock include sweating, a clammy feeling, paleness, hunger, anxiety, trembling, rapid heartbeat, fainting, weakness, irritability, change in personality or mood, and loss of consciousness. A person exhibiting these symptoms needs immediate medical attention.