Basically we get the tracks of PGR3 with brand new dynamic weather and lighting effects and those additions really do look NICE. We also get 2 new track locations that look great (in contrast, PGR2 had new location tracks one could download from live but that option never happened in PGR 3 or 4). We get to drive motorcycles now, but the general consensus is that the motorcycles dont really add much to the gameplay and might even be annoying since it doesnt feel like the game was actually designed with motrocycles in mind.
Frankly, given the above, the game was not worth 60bucks retail. At the current used price of around 30-40bucks, it might be a worthy purchase if you like the project gotham series. The game developer recently said it is "done" with Project Gotham Racing. This game sort of feels like they needed to make one more game because Microsoft told them they had to and this is what we got. It _is_ a solid racing game -- just dont expect much different from PGR3.