I am yet to meet someone (in person at least) that loves cars as much as I do, and GT5 fits my passion for cars almost perfectly, there's always room for improvement. Let's be honest, the only way GT5 would be perfect for each of us is if it had a version made especially for each of us, to our own particular taste. GT5 does a very good job of catering for so many tastes! Yes it's not perfect but it does have a very broad target audience.
I particularly love modifying and improving cars and GT5 keeps me happy most of the time due to the large amount of cars it offers. If it had less cars, it would have to have more parts to modify them with, in my opinion. I often spend at least 2 hours preparing every car I own to see which is best for a particular race series, then spent 30 minutes racing it.