I had a little surprise during my 2012 physical: I had a low GFR, which indicates poor kidney function. In addition, I had had a few aberrant readings in the docs office over the lost year of my blood pressure in the prehypertension phase. All of this pointed to my kidneys being under stress and perhaps even injured.
My doc quizzed me and was horrified to find I was taking creatine. Now I had read dozens of articles on creatine and all of them insist that it is incredibly safe and well-tolerated and does not affect kidney function, so I thought she was just operating on "old school" information. And perhaps she was, but I did find in my research that these articles, most of whom are on sites selling creatine, leave out several important facts about this supplement. And weight lifters like myself don't want to hear anything negative about creatine, because it is one of our star performers.
For those unaware, creatine has many incredible properties, which I document in my link on The Advantages of Creatine for Vegetarians, such as building satellite cells - one of the Holy Grails of bodybuilding - and boosting workout performance, etc. Unfortunately, to get those kind of benefits, you have to megadose. 5 grams is the typical maintenance dose and this is this is the amoung of creatine in a little over a pound of salmon! Unless you're working on an Alaskan fishing boat, you're not going to get anywhere near that on a daily basis. During the "loading phase" many men take creatine of 20-25 grams for a week or two as well. Some muscleheads like myself take 5 grams before and after a workout.
So let's look at just what risks creatine potentially poses in some men:
1. Raising DHT. DHT (dihydrotestosterone) is a steroid hormone almost as important as testosterone itself. It has a significant impact on libido, but is infamous for its role in male pattern baldness and BPH (enlarged prostate). All DHT comes from the conversion of testosterone via the 5-alpha reductase enzyme. This is why HRT almost always causes the prostate to grow: the body converts some of the newfound testosterone into DHT and the size of the prostate increases accordingly. It usually does not cause issues however.
Creatine supplementation was recently studied in healthy young rugby-playing males and found to increase by 56% during the 25 gram loading phase and 40% during the 5 gram maintenance phase.  In addition, the testosterone-to-DHT ratio increased by 22%, which indicates that there was increased 5-alpha reductase activity.
Does this mean that creatine usage could cause hair loss? Many men are low or lowish in DHT and so it is doubtful that they would be impacted. However, for men with more robust DHT levels, raising in another 40% is definitely not going to help matters. You can get your DHT measured - a doctor is unlikely to do it for you - at any one of these Testosterone Self-Testing Labs.
2. Kidney Issues. Creatine is widely used by bodybuilders and athletes all over the world. Few of them report kidney problems and so, at least in the short and medium term, creatine is clearly a safe supplement. A couple of small studies have even followed creatine user for up to five years and found no evidence of kidney dysfunction. 
Does this mean that creatine is universally danger-free? That is a big assumption. First of all, creatine increases the amount of urine by about 25%.  This just will create an additional burden on the kidneys. Obvioiusly, for healthy individual this does not appear to be a big problem however.
Long term users of creatine, though, should keep in mind that several cases of inflamed kidneys have been reported  and that studies on creatine usage have always been on the young and healthy. Long term usage by middle and senior-aged men may not fare so well. Furthermore, what about in men with type II diabetes, whose kidneys have often "been through the war?" Again, there are many large population groups who do not need and may not do well with this extra burden on one of their vital organs.
This reminds me a bit of salt in the diet, which will pull extra water into the arteries. This, of course, requires the heart to work a little harder, but is a burden that is handled fairly well by most people. However, in some sensitive people, it can create high blood pressure and other problems.
The bottom line is this: if you are going to use creatine, it would be wise to regularly check your kidney function in my opinion. Better safe than sorry, eh?
3. Creatine + HRT. As I have documented in my link on Testosterone and the Kidneys, HRT could cause issues in men with kidney problems. Some research shows that testosterone can cause cell death in the kidneys in certain cases, explaining why men have a much higher rate of kidney failure than women.  However, the good news is that giving men testosterone therapy is not noted for increasing kidney issues.
But I could not help but ask myself the question: did my GFR fall below range because I was on both testosterone AND creatine at the same time? Perhaps the two of these together created issues for me. (I was on testosterone cypionate at the time and my testosterone was peaking at about 1200 ng/dl at the time.) One further evidence of this is the fact that, after I gave up the creatine, my GFR rose again back within acceptable lab range levels.
1) Clin J Sport Med, 2009 Sep, 19(5):399-404, "Three weeks of creatine monohydrate supplementation affects dihydrotestosterone to testosterone ratio in college-aged rugby players"
2) Med Sci Sports Exerc, 1999 Aug, 31(8):1108-10, "Long-term oral creatine supplementation does not impair renal function in healthy athletes"
3) http://www.sportsci.org/jour/0001/inbrief.html , "CREATINE AND KIDNEY DAMAGE?", by Will G. Hopkins
5) Kidney Int, 2004 Apr, 65(4):1252-61, "Testosterone promotes apoptotic damage in human renal tubular cells"