Now when Vista was in RC stages and going to RTM yes, it was well worth it to download the RTM because of the large amount of bugs that were in the Vista RC.. (unfortunately the RTM didn't solve all of those bugs as we all know ). It would have actually seemed like a "better/changed" version of the OS in Vista's case.. I just don't feel that is the case with Win 7..
That's how I look at it anyway.. I'm not saying if you have the chance to download the Win 7 RTM don't, I'm just saying I won't be and that's my reasoning..
Yes the RC will work for a year.. March 2010 is when it expires and that's when the bi-hourly reboots will start.. However, the Final version of Windows 7 will in fact be released this holiday season (as confirmed by Microsoft) so you'll be able to purchase a final copy of Windows 7 long before the RC expires..