Metformin is one of the new blockbuster wonder drugs that improves insulin sensitivity and lowers blood glucose levels. It is widely prescribed to the general population with diabetes and women with PCOS. And it is sometimes prescribed to those with prediabetes as well, although I do not know how common that is. Considering the virtual epidemic of diabetes (and PCOS), this drug can be a life saver for many people, which is especially nice considering it has a relatively mild side effect profile.
Metformin also - and this is atypical for medications - may actually help with some of the "Holy Grails" of health:
a) Both an animal and human study show that Metformin may extend life span. 
b) Metformin lowers the risk of going from prediabetes to full-blown adult onset diabetes.  [Prediabetes is another health epidemic in our culture.]
I'm not much of a pharmaceutically-oriented person, but I admit these are impressive results. And we have had a number of men on the Peak Testosterone Forum using Metformin with good results.
That said, almost everything comes at a price and it looks like there may be a one with Metformin. There are now two studies out there that indicate that it may lower testosterone levels, or at least suppress natural rises in T at times:
1. Women with PCOS. Admittedly, everything is Opposite World with women in the area of hormones (and - I don't need to tell you - a lot of other things in life as well). Nevertheless, one recent study showed that in women with PCOS, Metformin led to a decrease in LH-stimulated testosterone.  Women with PCOS usually are insulin resistant and have visceral fat. Considering that men usually put on visceral fat first and a high percentage over about 35 are struggling with some insulin resistance, this is potentially portentous for males.
2. Obese Men. Does Metformin lower testosterone in men? It turns out that a couple of years prior to the above research on women, a study indicated, subtely, that this might be the case. Unfortunately it was subtle enough that I think it escaped recognition. First of all, we have to start by reviewing an even earlier study that showed something surprising: dieting in obese men could actually raise testosterone levels. This is actually counterintuitive, because normally dropping your calories significantly leads to a corresponding drop in testosterone. This can make dieting even more miserable for some men.
However, obesity will usually lead to a significant drop in testosterone in and of itself, something I cover in my link on Testosterone and Weight Loss. And the reason is that, as you gain fat, you also increase aromatase and this converts an ever-increasing amount of your testosterone into estradiol, the so-called "bad" estrogen. (Men do actually need estradiol.) Obese men often experience such a large decrease in testosterone from this process that they end up hypogonadal - it happens all the time.
These low T levels in obese men are undoubtedly the reason that, contrary to the general population, dieting for them actually boosts their testosterone levels somewhat. Yes, obese men can increase (their already low) testosterone through calorie reduction.
Now what does that have to do with Metformin? Well, a 2012 study found that obese men with and without diabetes who took Metformin did NOT experience a rise in testosterone as they should have.  This means that the Metformin likely suppressed the rise in testosterone and one can, without much of a stretch, assume that Metformin may actually lower testosterone in some men. The exact conclusions were:
"We conclude that metformin treatment combined with a hypocaloric diet leads to reduced FT [free testosterone] levels in obese nondiabetic men and to reduced TT [total testosterone] levels in obese men with type 2 diabetes. Increased SHBG levels may account for the decrease in FT levels in the former group."
CONCLUSION: The idea that testosterone could potentially lower testosterone or suppress it's rise in some cases is certainly preliminary. What I would think prudent for someone about to start Metformin is to measure testosterone, SHBG and possibly LH before and after. There are now many very inexpensive Labs for Self-Testing Testosterone out there. So I can't think of any good reason not to just pull your numbers.
CAUTION: If you are diabetic, do not change any medication or treatment without first discussing it with your doctor. Metformin can be life changing, so keep in mind that lowering your testerone a bit, assuming it even does, may be a more-than-reasonable trade off.
NOTE: If you are in the early stages of diabetes, there is a good chance you can actually reverse your diabetes. I cover two ways to do this in my pages called Reversing Diabetes with a Low Glycemic, Low Fat Diet and A Review of Reversing Diabetes by Julian Whitaker. In addition, testosterone therapy can have a powerful insulin-lowering effect, something I discuss in my page on Testosterone and Diabetes.
1) Medical News Today, "Metformin diabetes drug could extend lifespan", last updated: 8 August 2014, http://www.medicalnewstoday.com/articles/280725.php
2) Can Fam Physician, Apr 2009, 55(4):363 369, "Systematic review and meta-analysis"
3) Reproductive Biology and Endocrinology, 2014, 12:98, "Metformin induces a prompt decrease in LH-stimulated testosterone response in women with PCOS independent of its insulin-sensitizing effects"
4) Obesity Research, Nov 2001, 9(11):662 667, "The Effects of Metformin and Diet on Plasma Testosterone and Leptin Levels in Obese Men"