They love them as a treat; as jamncristian said, they are fatty and aren't a good staple. I've heard some people recommend no more than once a month; I'm usually somewhere in between (maybe once every couple weeks or so).
In my opinion, they provide no benefit, being essentially just blobs of fat. I'd say one a week is too many. Zero is a good number. My chameleons get them for special occassions only, like their hatch-date anniversary. Maybe 3 or 4 a year.