IGN just put up an article reviewing the GT5 demo. Basically, they said it didn't have a lot of content, but that the content it does have is really good:
http://ps3.ign.com/articles/105/1056254p1.html
Hopefully this isn't already posted somewhere else
http://ps3.ign.com/articles/105/1056254p1.html
Hopefully this isn't already posted somewhere else