I had a friend who had an interlock device; it was so unreliable, and triggered so many false positives that his sentence was extended because the DMV thought he was regularly driving drunk. What's worse is that when the device had a positive reading, he'd blow again, and it was negative. This was accurately recorded, and yet the DMV wouldn't recognize the impossibility of going from incredibly drunk to stone cold sober in the span of three minutes.

While I support the idea itself, the technology is incredibly unreliable, and since it only affects "criminals", there's no public demand for them to improve.