Having spent countless hours on each of the instalments of the GT series, the series has always had the philosophy that it was totally up to you how you wanted to go racing.
GT1 had its 1050hp Skylines, GT2 had the Escudo, and GT3 has the F1's. If you're totally focussed on finishing the game, you can get the cars that will let you do it quickly and easily.
However, you can also get the great satisfaction of winning races in 'inferior' cars. I remember winning the GT Gold Championship in a race-spec Honda Prelude I'd built by slipping past the leading JGTC NSX on the last corner. Very satisfying.
Now that I've picked up an F1 car, I'll be using it to finish the game as quickly as possible.
Once I've finished the game, then I'll start seeing if I can win races in inferior cars.
This isn't like Tomb Raider, where once you finish the game you'll never touch it again.
I finished the original GT very quickly (compared to GT2!) but continued to play it for over a year after completing it, for precisely the reasons I mentioned above.