At this point I'm pretty much just using focus to keep an eye on healers in warzones, but I'm not actually using a modifier actively. I've done it and loved it in other games, but currently so many of my abilities are already Shift+something that it's a bit of a fumble to get a third key in there. Thankfully I've done my time as a healer, and I won't be going back!
I do find that targeting in general feels a bit glitchy in this game sometimes. I almost always Tab target, but occasionally I get weird results (like targeting a mob behind me while there's 10 in front of me). I think WoW permanently ingrained in me a certain sense of what should get targeted, and unfortunately any system that works even slightly differently throws me for a loop. As a result, I invariably end up doing stupid things, like stunning some bystander in huttball while the other team scores.
What I'm most excited about in 1.2 is target of target! I find it particularly handy tanking or in pvp, because as soon as my target switches targets, I can take control of the situation (or at least attempt to).