"Which format are you interested in?"
One of the many advantages that XSI has over Softimage|3D is the support for a greater number of Image File Formats (jpeg, bmp, tiff, etc). This gave XSI a greater level of connectivity to a studio's production process. Basically, if your studio creates textures using tiff's then XSI can directly use that without any annoying and time-wasting conversion. I don't see how this should differ for 3D File Formats.
Ideally all file formats that are native for other popular packages. Like 3DSMax and 3DS, and Maya with OBJ. DXF would also be handy for importing CAD stuff. I think this will allow XSI to connect alot better with other packages, so artists are given a better opportunity to test out XSI's features.
Personally, DXF and 3DS are the important file formats. XSI should be able to import (and maybe export) in a similar fashion to other packages.