There's been all sorts of discussion about the negative effects of creatine - as far as I'm aware a lot of it stems from the fact that it raises your creatinine (a breakdown chemical - it's different, check out the extra letters
) levels. In medicine creatinine levels are used to measure how well your kidneys are working (high = bad) so creatine use can cause doctors to think your kidneys aren't working properly.
The other problem is that creatine use can lead to dehydration (one of its effects is to make your muscles suck up water), so especially when you first use it you'll find you need a lot of water to go with it. Sadly dehydration can mess up your kidney tests, and in some circumstances can mess up your kidneys - or make any kidney problems worse.
Lastly, to put things into perspective, the 'relatively short amount of time' creatine's been tested for is into the decades. Yes, noone's sure what will happen if you take it all your life - but for months/even a few years you'll probably be OK. Some people advise taking a few weeks/months off every now and again to let everything get back to normal though - if you're using creatine for the 'lets you train harder' benefit then that shouldn't lose you too much muscle.
So: stay off it if you have kidney trouble, drink lots of water with it, and make sure you tell your doctor you're using it if they ever take a blood test.
And yes, it appears to work - at least according to a good number of studies I've read and experiences from myself and various teams I've been on. Nothing magical but that extra few percent you're looking for. But you still have to train!