Word on the street is that the dudes keep their word for better or worst. You pay up on time, your files get decrypted. You pay late by 1 minute or don't pay at all and you're screwed, it just uninstalls itself and leaves you with a bunch of encrypted files.
If the files were important enough to pay ransom, the hackers may want to take a look at them as well; I would not be surprised if the virus sends a copy to the hackers while unencrypting the files.