About 1 in 3 of the nearly 8 million women in the US with infertility
have received drugs to induce ovulation and the #1 reason why women
don't ovulate (anovulation) is Polycystic Ovary Syndrome (PCOS). Clomid is the drug used traditionally to induce ovulation in women with PCOS and for many years now doctors have used a drug called metformin to treat women with PCOS in order to make them ovulate and get them pregnant.

In February of 2007 the New England Journal of Medicine published a study by Legro et al called: "Clomiphene, metformin, or both for infertility in the polycystic ovary syndrome."
The researchers randomly assigned 626 infertile women with the polycystic ovary syndrome into 3 groups and they then looked at the live birth rates:

The group which received clomiphene citrate plus placebo had a 22.5%  (47 of 209 subjects) live birth rate 
The group which received extended-release metformin plus placebo had a 7.2% (15 of 208) birth rate
The third group which received a combination of metformin and clomiphene had a 26.8% (56 of 209) birth rate.
The rate of multiple pregnancy was 6.0% in the clomiphene group, 0% in the metformin group, and 3.1% in the combination-therapy group.
The conception rate among subjects who ovulated was significantly lower in the metformin group (21.7%) than in either the clomiphene group (39.5%, P=0.002) or the combination-therapy group (46.0%, P<0.001).

With the exception of pregnancy complications, adverse-event rates were similar in all groups, though gastrointestinal side effects were more frequent, and vasomotor and ovulatory symptoms less frequent, in the metformin group than in the clomiphene group.

The authors came to the following conclusion: Clomiphene is superior to metformin in achieving live birth in infertile women with the polycystic ovary syndrome, although multiple birth is a complication.

An editorial by Dr.Guzick which accompanied the article states: Aside from the low but ever-present risk of multiple pregnancy, the use of clomiphene citrate to treat infertility in women with polycystic ovary syndrome is simple, generally safe, and ...more efficaceous than the use of metformin.