I would not call a game with over 5 million in sales a failure. If you split the total between November and December, Gran Turismo 5 averaged over 2.5 million sales in it's first two months of release. Granted, the bulk of that 5 million came in November.
The numbers in my source appear to be outdated and not reflecting any GT5 sales for December.
Source: http://gamrreview.vgchartz.com/sales/7678/gran-turismo-5/
Where all this hate is coming from (I am part to it) is an equal reaction to the pre-release hype of Gran Turismo 5. The hate is justified when you take into account: 1. The history of the Gran Turismo series, 2. Over $60 million in development costs, and 3. In development for 6 to 7 years. From this perspective, Gran Turismo 5 has underperformed. However, it has not been heavily bundled yet, so it is a bit premature to say the least.
The trolling is bad everywhere, not just here. The 2010 December NPD thread on NeoGAF has many posters cruelly one lining it with "GT5 lol!" and similar:
http://www.neogaf.com/forum/showthread.php?t=418652
Honestly, I think GT 5 is off to a good start. Once it gets heavily bundled for every PS3 sold in the Americas, Japan, and the EMEAA we will see it reach the 9 to 10 million mark. This being said, the time for it to sale bundled is not quite as long as GT3 because a lot of people expect Nintendo to end this generation come 2012.
Meaning Gran Turismo 5 will have the shortest legs of any Gran Turismo that has been bundled. Sure the numbers after the Wii 2 drops still matter, but not quite as much if this generation lasted until 2014 or later.