Well from Bioshock screenshot comparisons of DX9 v. DX10, there wasn't a big difference. Just a little more shiny and what not. But in the Crysis comparion screenshots, it was pretty noticeable.
Framerates though, didn't change much. I think it was maroon1, who posted a thread on that. Where the framerates weren't much different at all, between DX9 and DX10.