Those of you already running the near-finished version of Windows 7 can continue using it for free. Even if you miss the August 20 deadline Microsoft says they'll keep handing out license keys for a while, so you can always grab a copy from torrent sites – be sure to verify your ISO, though. You'll have until March 2010 before Microsoft starts forcefully shutting your computer down every two hours and then face expiration on June. That'll give you plenty of time to test Microsoft’s upcoming operating system past the official and final release.
One word of caution: As mentioned by Microsoft themselves and in numerous other sources, there's no upgrade path other than a clean install to move from the RC build to the final version, so that may be one reason to hold off and wait for the official release at this point in time. Should you decide to give it a try anyway, we have posted a guide to dual boot Windows 7 RC with XP/Vista in three easy steps, and another one to reverse the process.