If you don't have GT5 and haven't recently played any similar games, yep, get it. And although GT7 seems "around the corner", from what I know we won't see it for months. So, what are you going to be playing until, say, next Christmas? Tomb Raider? Metro Redux?
They're nice and swell games (TR and Metro), but if you need your "realistic racing", you've already got a PS3 and don't have any good racing games around, I'd say hit it. Get it and lose yourself in it's CaRPG action. And when GT7 comes around, just don't buy it right away if you're still bored from GT6 you'd be playing up to then. Let your copy gather some dust on a store's shelve, with its price dropping, playing other stuff, until you can buy it half-price and love every bit of it again.
That's what I do, anyways.