OEM versions are full versions but have acitvation restrictions, unlike the retail which can be used as many times as you wish. Originally the OEM versions were restricted to only one configuration of PC at the time of initial activiation, this has now changed to relax limitations on removable hardware such as graphics cards but does count for motherboard. So, in short, you can change your setup except your motherboard, which constitues a new PC and the reason the OEM versions are half the price of the full retails, you will no longer be able to use it.
Cracking methods seem to be of the same type as in XP...that is, a manual activation option (calling customer support), giving them the code generated by windoze, and then typing in the return code that the operative would have given you. Of course, it didnt take long for keygens to appear to do the same thing or even cracks to remove the necesity entirerly. You miss out on legitimate support (but there is no tech support with OEM anyway) and updates but serves you for pirating it anyway lol
Just been chatting to my friends and one says he has seen awful performance using the vista nvidia drivers on all but WoW and my other friend cannot even get 8800 drivers at all. They are all at early beta stage only and he is not willing to risk using them. So getting it early from overclockers wasnt such a great idea!
Overall they like the look and feel (once you get rid of the annoying dim-wit pop up 'help' boxes) and say it runs very nice. As for anyone desperate to upgrade, wait. One of them is thinking about putting XP back on already so he can play all his games again!!