Personally? I can have a waaaay higher luck than someone else, and have the other person get several more criticals on me than I do on them, and die because of it. In fact, that seems the usual for me, except when I'm facing monsters rather than players. -.= I'd not recommend relying too much on luck, because it is just that-- luck. It's good to keep it reasonably up, though.. Afterall, the occasional criticals DO make a difference.
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'I look like I need coffee, don't I?' 
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