The Hippo system is an interesting way to try and minimize the impact of opening knowledge and just "play chess".
However, the chess this system promulgates is largely of the dull, plodding variety. It has never caught on GM level because it's largely reactive - Black just sits tight and waits for White to overextend himself, launch a premature attack or blunder. Playing it requires patience and a subtle feel for the position, though; so it is a good choice against an impulsive attacker.
A classic example of the successful implementation of this strategy is Nezhmetdinov-Ujtelky (Sochi 1964). Both players were IMs, but White was known as one the most fearsome attackers of the time (Rashid had a 4-1 score against Tal, for instance). In this game Nezhmetdinov obtained a dominating position from the opening and went on to win the exchange with good winning chances. However, soon afterwards he lost patience and after blundering away a key center pawn went berzerk, sacrificing two pieces in quick succession.
The first sacrifice was dubious but playable, the second ruined White's game beyond repair. That was exactly what Black was counting on in that game.
Of course, this strategy had little chance against the more prudent players. After that game and a win over Kholmov Ujtelky's Hippo was drubbed mercilessly by everyone from Larsen, Spassky and Velimirovic to lesser lights like Suetin, Shamkovich and Lein.
I've been using the hippopotamus defence for a week now.
I think it's good defence for black and for white even some grandmaster's did use it like spassky with his match with petrosian in the world championship.
The game ended with a draw (2 games with hippo).
What Can you say about it?
The games are here and guide to hippo! :