I converted a large desktop application written in vb.net to ArcGIS 10. It was mostly smooth with a few problem. I hear the metadata is a mess but our application doesn't rely heavily on that. The editor is all different, I had to switch back to the 9 way of editing until I can get all that sorted out.
All that said, the application has to be ported, all the binding and licensing check stuff has changed. The language packs have changed. You can't just flip a switch and just have it work like 9.2 to 9.3. I found more problems going from 8.3 to 9.2, but that application was particularly bad.
