While other answers state that it makes sense for the malware authors to deliver on their promise to decrypt your files (I agree, it does) they offer no evidence. Anecdotal evidence is the best you'll get in the case, but it is not unheard of that the criminals will actually decrypt your files for you.
This New York Times article details how the author's mother's computer became infected with ransomware. Here are some selected sections to answer your question:
Of course, this advice arrives too late for my mom. And it appeared her payment had arrived too late as well: By the time I got home from Greenpoint, her CryptoWall ransom had been raised to $1,000, and the $500 in Bitcoins she had deposited had vanished. In a panic, she wrote to Mike Hoats asking for advice. What he told her sounded crazy to me. Use the CryptoWall message interface to tell the criminals exactly what happened. Be honest, in other words.
So she did. She explained that the virus had struck the same week that
a major snowstorm hit Massachusetts and the Thanksgiving holiday shut
down the banks. She told them about the unexpected Bitcoin shortfall
and about dispatching her daughter to the Coin Cafe A.T.M. at the 11th
hour. She swore she had really, really tried not to miss their
deadline. And then a weird thing happened: Her decryption key arrived.
But Mr. Wisniewski had a more pragmatic take. “From what we can tell,
they almost always honor what they say because they want word to get
around that they’re trustworthy criminals who’ll give you your files
Welcome to the new ransomware economy, where hackers have a reputation
Additionally Dark Reading cites two additional cases of police departments paying the ransom, and getting their files back.
However keep in mind, there are many types of ransomware written by many different programmers with varying level of skill. Some will simply not give you a decryption key, and I image some botch their whole malware and can't offer decryption keys. Other cases have been known in which the decryption key is stored on the victims computer, and you can recover your files without paying anything(example).