I've been putting off writing about this addon, mainly because it's one of the greatest addons out there, so it's hard to do it justice -- what doesn't PitBull do?
Well it won't win the game for you by itself, and its name doesn't really describe anything about it, but other than that it does pretty much anything and everything you could expect from a unit-frames replacement and then some. You can, of course, personalize your unit frames to your heart's content, more than any other unit frames mod I've seen, and that in itself is worth the download. On top of that, though, you get a ton of other features, all of which can be enabled for functionality or disabled for performance, whichever you like. Here are a few of my favorites: