from my expereince and from what i see in others, its made a big impact, but not as much as insulin has. i beleive insulin has made the biggest difference in our sport. combining it with gh allows the user to grow that much more and also stay very lean. on its own, even at higher doses, i dont think that it is as great as it is hyped on the net....at least not without insulin. you have to remember too that the guys you're talking about already have quite an advanced level of development. for a less experience user, say a guy doing a local or state level show, they can take far less than many pros and still do very very well. 

btw, i want to mention to anyone reading this that insulin can be a very dangerous drug and unless you are already an advanced athlete and have a SOLID understanding of how to use it, i would not suggest trying it out. for anyone who has been training on gear for less than 5 years, i would suggest they wait. for many people with less than superior genetics, they will end up with a lot more fat than muscle