I've only been able to put about 4,000 miles on it and have reached level 20 A-Spec, and level 17 B-Spec, and I still don't feel I've scratched the surface on this game though it shows me 65% done. The game is vast, but more wide than deep perhaps. There are pockets of depth, but there are many more wide fields to explore.
The game is deep in terms of handling, pictures, challenges, used cars, and wide in terms of item collecting, car collecting, car maintenance, and on-line racing. The "vibe" of the game is considerably different to Forza. Forza feels smaller except in the area of the auction house and party chat. And though Forza has more tracks listed, GT5 feels like it has more usuable tracks. The Course Maker adds greatly to the sense of more tracks in GT5.
The car list seems endless in GT5 even though it is true there are many duplicates. But it should be noted that none of the "duplicates" are truly exactly the same. They all have slightly different weights hp/tq and are in different states of freshness due to whatever mileage they have.
GT5 has a more serious tone to its driving experience, and yet, allows you to bump and grind way more than FM3. In FM3 the technical driving is less serious, but the bumping contact is way more sensitive. In this respect the two games are entirely opposite.
GT5 has a deeper driving experience that won't penalize you and end play because of a nudge, whereas FM3 is more sensitive to the things that might end the racing action, like bumping, time penalty, and sticky grass. Of the two, there is just more action for longer periods in GT5 especially with the huge grids and the ability to take an outside racing line. FM3 seems smaller, less diverse, and more isolated on track by comparison.