UUUMMMMM YEEEAAAAH! There is whole new world in GT for those without a wheel, I have a Fanatec GT3 RS V2 with All aluminum ClubSport pedals, and let me tell you that you will be quicker, quicker, if you know what I mean. Look at the top times in any racing platform, they ALL use wheels. Its not just the fact that you will be quicker but it ties it all together virtually to the real world, it makes the game tangible and far more entertaining. Get a wheel mate, you will be hooked!
And to the guy that says that you can be "just as good" with a controller, I'd like to see it. Maybe good, but not AS GOOD. The ratio of movement(wheel and pedals) with a wheel is far wider, therefore giving more control and accuracy. Don't let him fool you!
You wouldn't play Rock Band with a controller would you? You can BTW!