I think the last time APT borked a system I used unexpectedly was in 2003 and I was running Debian testing with parts of Sid thrown in: I expected it to fail from time to time. The fact it did only a handful of times was remarkable by itself.

The other time I remember was a couple months back when I added a bleeding edge Intel GMA driver to solve a bug I was trying to help to fix and it completely fubar'ed my X. It was easier to dump a package list to a separate partition and reinstall from the CD.

I never saw APT misbehaving unless provoked.