IMHO Pretty good, isn't good enough for billing. But that's a different debate.
I was told as much by the tech-sup at TSI, when I moved to cable, that as it was (and perhaps things have changed) they could not do accurate tracking, so until such time they could, it was honor system based. I was also told that if the metering was to be enforced, I would get an "official" heads up, via e-mail I supposed.
And though I have no written info for you, I can attest that I've spilled over a bit, a few times, without a hit on my bill.
So that's how I know that the honor system applies(d).