the focus range is huge on that lens. turning the the switch to "limit" it limits the available focus range to either the nearer half or the further half (depending on where you are in the focus range when you turn the switch on). like lots of macro's this lens also hunts, a lot. by cutting down on the available range it doesn't search through the entire focus range to try and lock focus. i usually have mine limited to the "far" end, and this gives a normal-ish (non-macro lens) focus range. although it still hunts, it makes the hunting (and therefor AF) faster. only when i'm doing a macro shot do i switch to the full range.
Cliff notes: portraits, limit switch on (while in the far end of the focus range). macro, full.
enjoy the lens. this is easily my favorite lens (even above the 70-200 vrii). it's disgustingly sharp even wide open, and with beautiful bokeh. it suffers a little from CA and the colors with the nikon auto WB generally come out very cool (this seems to be typical of tokina), and it focuses slow, and is awful tight on a crop.... but the shots it makes are beautiful.