Although Catalin Taranu beat the computer in a 19x19 configuration with a nine-stone handicap, the Go Master nevertheless rated the IA system as 'approaching Dan standard' in a performance that promises some formidable battles to come between man and machine. Dan standard would be a ranking of 2100 versus a 2830 ranking for a 5th professional DAN player and 2940 for a 9th professional DAN player.
From a paper: Solving Go on a 3x3 Board Using Temporal-Difference by Learning
Choon Ngai Tay
Computer Go is one of the biggest challenges faced by game programmers. One of the reasons that it remains unsuccessful is due to the enormous search space. The size of the search space for a normal 19 x 19 Go board, is estimated to be 10^170 states, whereas the search space for chess is 10^50. The size of the game tree for Go is approximated to be 10^600 compared to 10^123 for chess. However, this is not the main reason as the small 9 x 9 Go board has a search space of 10^40 and a game tree size of 10^85 is also unsuccessful. The main reason is that until now no one has derived an evaluation function that accurately describes the intermediate Go states.
The complexity of different size games of GO at wikipedia.
Size Game tree complexity of average game length
19×19 3 X 10^511
21×21 1.3X 10^661
TAO : Machine Learning and Optimisation site
the University of Alberta helped improve the Go system. The University of Alberta had researchers that helped to solve checkers.
A research paper on how the MOGO system works.
Website of one of the french researchers
Computer GO at wikipedia
A research paper on the difficulties of programming GO
Complexity table for many games at wikipedia
Statistics on even GO games (19X19 board)
Professional 4th DAN, 5th DAN, 6th DAN players generally never lose to some 4 rankings less than them. A 6th pro DAN player generally never loses to a 2nd pro DAN or lower player. The better players tend not to lose to even slightly weaker opponents. The 6th pro DAN players tend to not lose to even 4th DAN pro players and only lose 15% of the time to 5th DAN pro players.