Leaving aside the fact that occasionally, AdWords Experiments Can Be Misleading, on balance, they really are the best thing since sliced bread.
Have a look at the screen shot below for an example of how useful they can be :-
Looking at ‘Google Search’ in isolation, you can see that for for this campaign – the client is getting not only a better ROI, but is also making more profit with an actual CPA of £54.31 compared to a CPA of £82.67.
This sort of information was a lot more difficult to come by before AdWords Experiments arrived, so fair play to Google for providing us with something that allows us to see more easily where we are giving Google too much money!
If you are getting reasonably high volumes of traffic through certain ad groups / keywords, then it is definitely worth looking at AdWords Experiments. Here is some more info from Google :-
And here are some videos if you are that way inclined:-