My two started using a bunk bed when my oldest was about four and a half. We knew we were taking a chance, but it was either that or have them keep sharing a single bed. And the local children's aid gets really squirrelly about opposite sex siblings sharing a bed.
So we bought the sturdiest bunk we could find, and the oldest went up top. She never had any trouble climbing down from the top for midnight bathroom breaks. However, she did fall off the top at least once, maybe more, while playing. For some reason she decided she had to touch the opposite wall, couldn't reach and overbalanced.
We always said it's a good thing kids that age bounce! She had some bruises, but managed not to kill herself.
That said... do I think it's a good idea? Not if you have other options. If I'd had two little girls, I'd have let them sleep together a few years more.