I'm curious as well. My NB kit for White House Honey Ale provided these so I used them for the kit and the remainder for my next partial mash. They are coming up on 3 weeks in the bottle so I should be able to tell you how well they carbonated in a week or so.
They are damned convenient - I can bottle straight out of the primary. The only potential problem that I see is that is a fixed charge of sugar and really doesn't allow for any accounting for beer style. I measured their mass once; depending on what type of sugar they are made of they (on paper) seem to provide enough sugar for even high volume carbonation styles.
