GT6 introduced some fictional cars in collaboartion with Red Bull, such as the Red Bull X2010. Did you find this to be in line with the rest of the game, since it was quite a realistic racing game? I found the performance to be too broken to my liking.
I felt like it was just a marketing gimmick. The cars didn't even drive that nicely, and I even noticed a few bugs with them. Gran Turismo is known for its realism, at least for me, and when they went and added in this line of cars I did feel like it was out of place.
The concept cars were fine, although they did have their own issues. I wouldn't consider it a huge gimmick, although you could certainly make that argument. It was a decent addition to the game that could have been made a little better with some more attention.
The X-series cars in Gran Turismo 6 were not bad. Now better ones exist. These games are always upgrading and remodeling....
To be honest I wasn't really a fan of the X series cars in GT6, and I felt that the only reason they was in there was to 'bulk up' the amount of cars that they could include in the game. Personally I would like to see more everyday cars in there in stead, cars that we can actually go out and buy in real life. I know that for a lot of people choosing between a string of Honda's or Toyota's might not be that exciting, but I'd rather have them than a selection of cars that we have no chance of actually driving or seeing perform on a race track.