I don't really care how it is integrated.
I personally loved the Limit Breaks of FF XII Revenant Wings, they are just the first thing coming to my mind when I hear "Limit Break". Someone already said the MMO world does not stop while you have your 30 sec cutscene of beating the mob to smithereens, but what I liked about it were the different LBs you had, because every class had some special LB only they could do. But even if you used a mage's LB on a mob which was immune to magic, it got no dmg. So LBs were great, but not totally overkill.

What I'm trying to say is..actually, I don't care how it is done, I just want some x)