Ok, so please don't flame me for this--just trying to see if I'm in the minority here.
I bought Need for Speed: Shift for PS3 shortly after it came out. It's an impressive game, and I bought it because it got reviewed well, and most of my friends at work bought it for their PS3s. I love the graphics, I love the look of the cockpits and car exteriors, and they absolutely NAILED the internal cockpit view. In my opinion, NFS:S was the first game to do this effectively.
I also had a chance to play the full retail copy of Forza 3 and I was very impressed. Forza 3 has the slight edge graphics-wise over Shift. The game play is fairly similar--I have been told by car sim nerds that Forza 3 emulates the sound and feel of each individual car better than Shift, but I wouldn't know because I'm no car nut. All in all, I was very impressed by Forza 3. Even the environment outside of the race track looked nice. But in the end, I figured it's not a big enough difference. Need for Speed will do; I'm not a big racing freak. When I feel like racing, I have a perfectly good game in Shift to satisfy that need.
Today I played the Gran Turismo 5 demo. I was surprised at how not blown away I was. I doubt the retail release will look much better than the demo, so I'm assuming that what I saw is what I'm going to get, especially after this long of a development cycle. The environment looked really dull compared to NFS or F3, the graphics looked bland and jagged, and overall, I didn't feel compelled to even finish playing the demo. I was bored, and I can't believe they are taking this long to release this game. I have a feeling it might be a turd.
ALL THAT BEING SAID, I'm sure it will sell well and I hope it pushes hardware for the PS3, which is an awesome console. I just don't necessarily think that means GT5 is going to be a good game. After all, people buy Halo in droves. ;)