I've been highly suspect of them for years (I may have used them a few times, but it has alawys been very discriminate and I've verified via other sources when doing so), and a recent controversy over their "fact-checking" of President Obama's State of The Union seems to suggest my suspicions were well-founded.
They rated two things he said as about jobs as true but ranked the statement as half-true. Then, under outcry and controversy, they backtracked and upgraded both Obama's absolutely true jobs statements as mostly true.
Again, they found both his statements as quantifiably true, but they still ranked the statement as mostly true.
Rachel Maddow responded to PolitiFact's bullshit thusly:
"PolitiFact, you are fired. You are a mess. You are fired. You are undermining the definition of the word 'fact' in the English langauge by pretending to it in your name. The English language wants its word back. You are an embarrassment. You sully the reputation of anyone who cites you as an authority on factishness let alone facts. You are fired."