There is only one known issue with the XenForo core and PHP 5.4.
Quite simply, this manifests itself in a template error that only displays in templates where it is possible to upload attachments.
This was reported in XenForo 1.1.2 and a "band-aid" was applied for XenForo 1.1.3.
The "band-aid" in question is simply that the template error is only visible while Debug Mode is enabled. This will put an even bigger emphasis on only enabling debug mode for specific users or, indeed, never on a live board:
http://xenforo.com/community/threads/php-5-4-bug.29835/
My only question is, yes, it is only visible with debug mode enabled... but actually is it deeper than that and only visible to Admins/Super Admins? If it's only displayed to Admins it's really a big non-issue.
That's the core, anyway...
You may find compatibility issues with some add-ons but specifically I've only ever seen one and it affected a number of add-ons. That would be almost any add-on that included a forum selector in the options. I've still not got my head around it but it used to cause array to string conversion errors. I think most add-on developers have resolved this issue now.
I've built my own forum and usergroup selector that definitely works on PHP 5.4.
So really, short answer: from a user's perspective it's completely fine to use.