Having just left a comment on Daniel's blog about how preempting with a semi-balanced hand is a bad idea, I went to go look at some of the hands I'd played on BBO earlier in the evening.
And lo and behold, I had preempted. With a semi-balanced hand:
This time, the preempt worked out well, though, because it warned partner off from bidding. The juicy double yielded +300 which was worth 82% of matchpoints even with a less-than-ideal defense. Most everyone was in NT our way and going down.
[You can see our defense by clicking on Next; you can see ideal defense by clicking on GIB]