Never used it. I have used other permissions management tools and firewalls without problems, but of course it may depend on precisely which app you block and what you block it from accessing. The description at XDA says that for some things you can return random values, which may reduce the risk of failure.
It needs root, of course, but that means you can take a nandroid before testing it, and if it causes problems just restore the nandroid (I've not read in enough detail to see how cleanly it can be disabled, but if it's making framework mods I'd nandroid first anyway just in case there's some incompatibility).