I'm gonna have to have a look into GTR, it's looking kick ass from everyone's posts. Btw, go the images, uber cool racers in GTR, decent machines in DTM, boring boxes with wheels for GT4. w00t.
*Prepares to be beaten by boring box lovers*
My vote will stick with GT4. Regardless of the quality of other racing games, or even how much fun they are, none have the longevity of GT. Sure, NFSU was far more fun and arcadey, incorporated upgrades and mods nicely, but even 112 challenges ran thin. Le Mans had me bored after winning everything in a matter of days, even a real 24 hour race isn't that special. Only GT keeps me coming back for 3 years to win everything, GT4 will be no different I'll wager.