If by effective you mean preventing another terrorist attack on the CONUS, then you may be right, or wrong. Truth of the matter is we don't know for sure what was prevented or disrupted and what role the folks at GITMO had or didn't have in the prevention or disruption, if it did or did not happen.
If by effective you mean IAW the Constitution that he was sworn to uphold, protect and defend against ALL enemies, foreign and domestic, maybe, maybe not, depending on how you look at it.
Few, even the arch-neo-more-to-the-right-than-Barry-Goldwater types on this board, would willingly point to what we have done at GITMO as a shining example of what is known and revered around the world as American justice. GITMO is a great stain on the National morality. We are expected to be, and should be, better than that. Keeping people behind bars for years on end without the benefit of counsel or charges are things authorized by two-bit, tin-horn dictators, not the President of the United States of America. The greatest example of the level of justice present in a society is the justice it provides to those that would seek to destroy it.