From the book "Losing It" by Laura Fraser, page 107-108 (paperback edition):
Three studies have shown that chromium has no beneficial effects. In one study of football players undergoing intensive weightlifting training, there was no difference in either body composition or strength between those who took the chromium supplements and those who didn't. The only difference between the two groups was that the men who took the chromium had five times as much chromium in their urine as the others. In a study at the US Dept of Agriculture Human Nutrition Research Center...35 men [were] put through a rigorous weight-training program for 8 weeks. "We were unable to find any significant effect of chromium in gain of muscle mass or loss of body fat...the bottom line is this stuff is being sold all over the place, and we really don't know that it has any beneficial effect at all."
In 1995, researchers at Dartmouth College and George Washington University Medical Center did a study on chromium picolinate which indicates that too much of the mineral may actually be carcinogenic. The researchers tested the effects on cells taken from a hamster's ovary. The chromium picolinate caused damage to the chromosomes in the cells, which is often an indication that a substance could cause cancer.
At this point, it isn't certain that chromium supplements are dangerous...but just as uncertain is whether anyone needs to take chromium at all. The only thing you can say with any confidence is that taking 200 mg a day of a trace mineral isn't likely to make you lean and muscular. It's more likely to give you expensive urine.
In 1997, the Federal Trade Commission ruled that Nutrition 21, which holds the exclusive patent on chromium picolinate, can no longer advertise that the substance burns fat, causes weight loss, increases muscle mass, reduces serum cholesterol, regulated blood sugar levels, or can treat or prevent diabetes. There's simply no evidence to support those claims.
There was also an interesting article in the December 2000 issue of the University of California Berkeley Wellness Letter - however, I don't subscribe (I read it at the library - I've got WAY too many books around the house right now...)
My opinion - if it doesn't work for weight loss (and it appears that the only studies showing that it MIGHT work are the ones which were done by Nutrition 21) and there's a chance of too much causing cancer (a concern to me since I have cancer risks on both sides of my family) - it just ain't worth the risk. Even without the cancer risk, I have better things to spend my hard earned $$ on than another pill that doesn't work.