I had my DNA tested by 23andme about two years ago. It was free because I have been diagnosed with Parkinson's.
Although they have now launched the regular testing service in the UK (cost £125) the free testing for PWP is not currently available in the UK, until they get approval from the relevant Ethics committee.
You get access to an enormous amount of data which will suggest that you have a higher than average chance of developing "Disease X" and a lower than average chance of developing "Disease Y". You certainly should not act on this information without consulting your GP or other medical professional.
You also get a lot of data about your ancestry which may or may not interest you. For example they tell me that I am 2.8% Neanderthal.
Probably the most famous example of someone acting on genetic information (I think from 23andme) is Angelina Jolie who had a double mastectomy when she discovered that she had a specific genetic mutation with meant she had a more than 80% probability of developing breast cancer. But I'm sure she discussed this with medical people first!
And this is unusual, usually the tests say that your probability of getting "Cancer A" has increased from 1.25% to 1.8%. To which your first response should be "Don't Panic" and your second response should be "OK are there any lifestyle changes I can make to reduce the other factors which lead to this condition?"
But its one more piece of useful information. For example, my DNA results suggest I have a higher than average response to warfarin so, if my GP suggested I needed to take this drug, I would discuss the 23andme report about this with him and we would probably start with a more cautious dose.
If you want to know a bit more about 23andme I suggest you read this (7 part) article: