Pretty general question I think. I’ve been thinking about using it but maybe it isn’t worth it.
No real cites, but I take Creatine Monohydrate and have seen no studies indicating that it’s harmful.
Wikipedia indicates that it’s safe as well: Creatine - Wikipedia
The only problems are that you will get incredibly thirsty. Drink enough water and you pee a lot. Don’t and you get muscle cramps. Just a couple grams per day or most will come out in your urine. I FEEL stronger when I take it, but I also think I can run faster if I have new shoes on or high black socks. 
YMMV.
Here’s a past thread on the subject: http://boards.straightdope.com/sdmb/showthread.php?t=360493&highlight=creatine
Personal anecdote: A semi-friend/acquaintance in peewee and high school wrestling died while using creatine. Creatine, from what I understand, draws fluid away from the muscles (this is probably over-simplified and the doctor/girlfriend would have a fit), which tends to lead to heart and kidney failure. But, properly hydrated, I’ve never heard any complaints. Wrestlers (back when “sucking weight” was still allowed) tended to dehydrate themselves a lot, and the added dyhydrative qualities of creatine didn’t help.
I have used it, and it is quite effective. You will feel a much better muscle “pump”. But, as others have said, you will get BAD muscle cramps if you don’t drink plenty of water when using it.
Another personal anecdote -
I tried it. I bought a batch, went on it, and got sick almost at once (possibly dehydration, says my nephew, who says the same thing as GiantRat). Finished the bottle, and am currently on the phase of the experiment where I see how I react to not taking it. Last Tuesday I set a PR in the bench press (for my current age range).
Totally worthless as far as real evidence goes, but FWIW.
Regards,
Shodan
I also would add that drinking lots of water is important, but that is true anytime you work out. Water is to muscles like air is to tires. Stay well hydrated!