About 2500 miles so far. I've noticed it seems to freak out about semis in adjacent lanes as I come up on them w/ TACC. It's not every time but it's happened about 6 times so far. Once even going so far as to trigger the collision alert chime(!) - yet the semi was completely in the other lane and never moved over in any way. It's like the radar or sensors suddenly goes 'oh crap - there's like two cars on top of each other (the cab and trailer) sticking into our lane and they came out of nowhere(!). With my morning/evening commute being on a 4 lane city loop with traffic usually moving at a pretty good clip, these events seem to happen almost once daily.
I use TACC *a lot* but I've gotten really careful around semis in certain situations.