Well, it's been studied for many years.  There are many Life Extension drug (Smart Drug) users out there.  I am just the opposite, I'd much rather take a life extension drug than a vitamin. 

http://nootropics.com/

The only side effect I have witnessed in about 10% of my patients is stomach upset and diarrhea. I've found, however, that almost everyone can build up a tolerance to it. I suggest that my patients start with a low dose, as little as one, or even a half tablet per day for a week or two, and then gradually increase the dose as they develop a tolerance. There is a theoretical problem of lactic acidosis in patients with impaired kidney function, but in practice, I have never seen this, despite monitoring thousands of patients on Metformin. Nevertheless, I believe that anyone with impaired kidney function should take Metformin only under the care of a physician, and should have their blood levels of lactic acid checked periodically.

http://www.antiaging-magazine.com/interviews/interview_1.html

Metformin is chemically very similar to Phenformin, an anti-diabetic drug which was discontinued in the United States by the FDA in 1976. The reason for Phenformin's removal from the market was because of a number of excess deaths in diabetic patients, caused by lactic acidosis. These deaths were invariably due to the continued use of phenformin by diabetics who had compromised kidney or liver function. No deaths were ever reported in patients who had normal kidney and liver function.

Phenformin's removal from the market was greatly lamented by most diabetologists at the time, one of whom once told me, "Phenformin did everything! It lowered blood sugar, lowered insulin, reduced blood cholesterol and triglycerides, stimulated immunity, and even resulted in weight loss!" He blamed the excess deaths associated with phenformin's use on the "stupid docs" who continued to use it in the face of abnormal liver or kidney function. Metformin is similar to phenformin in its beneficial metabolic effects, but it is much safer, in that it has even less propensity to cause lactic acidosis than phenformin.

http://www.antiaging-systems.com/extract/metforminweight.htm

SOURCE: Archives of Internal Medicine October 9, 2006 Dr Ralph Cinque wrote:

If I had diabetes, and if I was uanble to manage it through diet, exercise, and dietary supplements, such as lipoic acid, cinnamon, chromium, etc., I would take Metformin. Also known as Glucophage, Metformin is the pharmaceutical equivalent of a naturally occurring herbal substance known as Goats Rue. There has been a great deal of research done on Metformin, and it has demonstrated life-extending effects - at least in animals. Some of this research was done by our own Dr. Ward Dean in conjunction with the late great Russian physiologist, Vladimir Dilman. However, more recent research has been published by the Life Extention Foundation showing the life-extending effects of Metformin. These results have impressed some people enough to start taking Metformin as part of their life extension programs, even though they are not diabetic. I have not gone that far, but in any case, I don't think there is any question that Metformin can be, and often is, a useful and worthwhile intervention. Regarding the problem with B12 depletion, it's a good thing they found out about it, and taking a high oral dose of Vitamin B12 is sure to take care of it. All in all, I still think that Metformin is the best of the oral diabetes medicines

I also take hydergine, which is among the world's most popular life extension drugs.  i use it.  i started on Life Extension Drugs over 20 yrs ago.  Hydergine has been used in the states for many years.  They use it when someone dies, they inject it.  It can save the brain from damage for like 45 mins or more.  it oxygenates it.

http://www.smart-drugs.com/Hydergine-WardDean.htm

http://www.lef.org/prod_hp/abstracts/hydergine.html

And, I take Idebenone (and a few others): 

http://curezone.com/blogs/m.asp?f=1184&i=139