http://www.cnn.com/2006/TECH/internet/06/23/france.google.earth.reut/index.html
http://www.geoportail.fr

I can't access the site, so I can't comment on how it stacks up. According to the article, the imagery of France is supposed to be more detailed than on Google's services.

and it IS more detailed.
U don't see the difference on the big big cities, but on the small towns you really see the difference

I can't access to Geoportail.fr too (their server is saturate, millions connections...) but I did an article few weeks ago about a site wich use the same database of geoportail.fr
and I did a comparison
http://www.zorgloob.com/2006/06/un-avant-got-de-goportail.asp