Its a weird thing, I always thought GT5 (and FM4) is a really good game, but I've gotten much more enjoyment from Horizon. From what I gather by reading comments about Horizon over at gtplanet and forzamotorsport, it just seems a lot of the core FM fans either weren't interested in the game to begin with or simply disappointed with the game if they did buy it, and the disappointment seems to range from not a big enough map to not enough cars or they simply don't like the feel of the cars, no tuning, 30fps instead of 60fps, etc.
Apparently, the game just didn't catch the eye of the general public and is largely dependent on fans of the Forza Motorsport series.
Personally I love it, I bought the season pass just to show my support for the devs, because I would love to see more content and a sequel.
