AlphaGo proved that Go is much more complicated that we thought, and even after perfecting this game for hundreds of year, humans are still easily defeatable by AlphaGo using very simple-looking moves.
How strong exactly is the Master version of AlphaGo? The professionals playing with it had various reactions and gave different estimations. Personally I believe that the latest AlphaGo version can give 3 stones handicap to top pros and still win.
Here are some interesting moves from the 60-0 series - the marked move is AlphaGo's:
For more complete and up-to-date information, see Sensei's AlphaGo page.