Once, there was some weird glitch with my iCloud account, and my Mac was locked without any reason.
The PIN code consists just of 4 numbers (10000 values total), so I solved this problem with stupid Bruteforce method.
Spent several hours on that, because every 5 tries, you need to reboot your Mac to reset the "waiting" counter,
and reboot takes almost 1 minute. Was not bored at all: there was a PC near me and I used it for Internet browsing.
But, after this situation, I am really worried about this feature: since the real thief could do the same thing as me
and unlock the Mac in less than one day, then this feature does not bring any security at all!
Should be more numbers, or letters as well - otherwise, it just too easy!