My dog just love, love, loves these. The mere mention of the word green has her begging for one! Greenies usually last her about a minute and a half to eat, probaly because she should be eating the large ones instead of the regular ones. Regular is cheaper though, and she doesn't really seem to mind.
Greenies freshen dog's breath, clean teeth, and appearently get rid of tartar. We buy the bags that have 12 greenies in them, but we used to buy the boxes that had more.
The only problem with greenies is, they're so expensive for dog treats. My dog used to eat them almost every day, but now with the economy she gets one every week or so.
She's been eating these since she was a puppy, and she's now 8 years old. We haven't seen any problems with them, and her tummy is pretty sensitive. A little too much food and a little running around, and her food comes back up.
Overall, I would reccomend these to any dog owners who are tired of putting up with doggy breath, and I think they even have some for cats if you want their teeth clean too.