Well for me I pre-ordered a few months ago. First, cause I love GTA, and second of all I needed to know I had it because anything can happened before launch.
It was a very good decision as there is no way I could afford it now, maybe not even for another 2 months yet.
Now imagine how awful it would be if people were going 'WOW, Incredible!, Rockstar is made up of Gods, THis game is ******* awesome! etc..
You'd be upset, because you don't have the game, you can't get the game and people are having fun while you sulk.
So pre-ordering GTA V was such a good decision for me, and whatever happens I will not regret it
And it will NOT run like GTA IV launch, I think 90% of us are fairly confident on that