With regards to amazon or similar review sites:
To be entirely hypothetical here, currently there stands over 200 verified (people who have at least purchased the console) negative reviews.
If we assumed Sony or Microsoft, or a company they hired, such as a reputation management company, were handed a lump of cash to tarnish the competitions reputation, it could cost them just 79,998 usd in order to buy and then review (badly) 200 units, vastly effecting the overall review score - in the grand scheme of things, 80,000 usd is a raindrop in the puddle of advertising expenses.
On top of that, Not everyone buys consoles for themselves, or indeed, buys consoles to keep, We have the presence of scalpers who buy consoles purely to sell on, such people could very well have no intention of keeping a ps4 or xbox one, and chip in their bad review on purpose, after all, if someone posts up an ebay add saying "tested and verified working" then they might even get some additional money.
Then theres instances where family members or friends will pick up consoles for other people, even if they dont actually like the brand of console they pick up, again these people, while buying the console, would have no real reason to leave positive feedback.
Just because a person is verified, does not instantly mean they are a genuine source of information.
I can see this occurring on the Xbox One reviews when it launches too.
We live in a connected age where online image is more important than ever before, It's only right then to assume that these companies will resort to underhanded tactics where possible to give one another the upper hand, it happens during elections, it happens during wars, there's no reason to assume it will not happen with very important products from two companies line ups.