All that can to be runned will be cracked. There is no Uncrackable protection's.
Starforce use Ring0 based on Protect.dll, that mean's that you can't debug it with ring3 debugger's (Olly, etc). But all ring0 protection's (like "Xtreme Protector"), if you know, is often unstable on different PC's configuration's and may crush user system. So you can decide yourself use or not this protector.
PS: MS is restrict all action's, like Ring0 protection's, in there system kernel, and I think that soon using protector's, like this will be illegal.
Try to test your game protected with SF under the StarFuck sometimes this tool can bypass this protection.