I just ordered the pellet version of these -- active ingredient in both is (I believe) Bacillus thuringiensis sub. Israelensis. Not entirely sure what this little bacterial friend does to the larvae, but I believe it has something to do with blocking up their insides to the point that they can no longer feed.
I've been fighting fungus gnats in my houseplants since last June or so. Though that I finally got rid of them over the winter, but I think it was just the cool weather keeping them hidden. They don't seem to have found their way into my girls yet, but will be treating all of my plants with this soon. Will give an exciting update once I can confirm/deny efficacy.