Surprisingly good
Our two black labs are 8 months and 5 years old, and have small and large Denta Stix respectively. The dogs destroy them very quickly, and within 20 seconds they're gone - they certainly aren't treats that go on and on, contrary to the TV ads' claims.
I was at first very sceptical that they would have any benefit whatsoever, and had intended to run the boxes down using them as treats and training rewards, before switching to something a bit smaller (and therefore cheaper).
However, well before the boxes were half-used, we noticed that plaque on the older dog's canines and molars was greatly reduced, so have decided to continue using them as a one-a-day treat.
The dogs get their Denta Stix in addition to their normal amount of food, and this has not adversely affected the weight of the older dog (an un-neutered male). They rarely have other treats, however.
Ultimately, the dogs adore them, they're not too expensive, and they do seem to do their job. I would expect faster results with dogs that chew them instead of inhaling them!