Well, I don't have it yet, but I'm looking for some reviews.
In particular, does the slideshow give you any control over transition effects? It always annoyed me that I had to import my pictures into iMovie and make a movie so that I could have transitions when the the iPhoto slideshow, the Desktop, and the screen saver all have nice transitions to begin with. Perhaps this would be difficult to do as each photo is now a chapter enabling you to manually go through the slideshow. I don't actually know if this is the case, just guessing.
Also, clicking on iDVD in iPhoto 2 will import the photos into iDVD 2. iDVD 3 is not required for this.
thanks,
kman