My problem with it is that it's not in GT5 and it isn't handy when playing GT5. I don't happen to have a computer right next to my racing wheel, and I also don't see me keeping a computer running the whole time I'm playing GT5 just for the sake of checking MyGranTurismo to see whether or not I have a certain car. If I don't have a computer running right beside me to check the site, I have to make some sort of note of what cars I've bought and then transfer that information to the site. So, the site is of no real value to me, and I just keep track of my cars there for the sake of doing it rather than as any real help to me in GT5.
What works for me is to simply print out the car list and make a mark next to the ones I already have. I actually update MyGranTurismo based off of the notations on that list.