I'm not sure why this needs to be stated so often: GT5 is a success by gaming standards if we consider it a typical game. It's an enormous success, in fact, if it has already shipped 5.5M copies.
If the standard is the GT franchise, then the sales are, at least, underwhelming.
If the standard is "Sony's flagship franchise," then they are also underwhelming.
If the standard is profitability, then GT5 is unquestionably profitable, and likely profitable by a wide margin. Also good.
The problem with these arguments is that no one is precisely wrong about GT5: it's entirely possible for one person to say the sales are good while another says the sales are bad, as they're coming from two different perspectives.
Sometimes, some perspectives are not legitimate. For example, if GT5 sold 1M copies total, and you said, "well, that's more than any other racing sim on the PS3 that had a prologue has sold," I would consider that a flatly invalid perspective. If God of War 3 had sold 9 million copies and someone said, "that's not as good as Halo," that would be a flatly invalid perspective.
But in this case, I think there are several legitimate perspectives, and they don't allow for an absolute consensus of opinion. In some ways the game did relatively poorly. In other ways it did excellently.
To wit: GT5 is a game that sold extremely well on absolute terms, but not very well given the series pedigree. The end.